I think I know the answer to this but just wanted to double check.

If I use the tuning file with a game I was previously using Retuner's in-game settings for, which one now takes priority? The tuning file or the previous settings? Do I need to use the "Reset to Default" option in Retuner to enable the tuning file to take effect?

In-game setting's always win. Yes, you'll need to reset for the tuning file to be used on values already customized in game.

Ah right, thanks Chain. I was wrong in thinking the tuning would take priority then. Silly me.

I always think of it this way. The place where Retuner adds more overhead to the game (as we've always been warned, but I've never noticed it to be all that intrusive) is when the game clock starts moving smoothly. By that time, any standalone tuning files had already long since loaded. So Retuner trumps all of those as it kicks in so much later in the loadup process. Not a very technical explanation, but just in case that helps...

Oh I see! Thank you igazor. Yes that does help me understand. I always thought the overhead from Retuner occurred much earlier on in the load-up process.

Hmmm...so that gives me a dilemma though because I thought I might be able to adjust things further with the in-game settings and then export a new tuning file but I see now that won't be possible. Oh well, can't have everything I suppose.

Huh..... why not. It may not be the same as what you thought it would be (and if the export file doesn't consist with the settings from the Tuning File), but you can definitely work on your existing file and do some maintenance on it afterwards. (If the export does include the settings in your Tuning File it would be solved).

Retuner will see the Tuning File for the adjust parts as default settings, so if you add more in the in-game settings, all you need to do is check whether some of the "new" in-game settings already exists in your Tuning File based on the export. If so, change them to exactly what the export says in your Tuning File and delete it from the export, till you are sure there are no duplicates, than copy the rest over from the export to the tuning file in the right sections, just what I wrote earlier with the ChickenCoop.

Are you expecting to do over a 100 changes more? To my guesstimate you already caught almost everything you wanted, so the list in the export should be just a few.

Five seconds after I posted my last message, I suddenly thought, there's no reason why I can't keep my existing in-game settings in my current save (and continue changing them if I need to) and also leave the tuning file in the Mods/Packages folder so it kicks in whenever I start a new world. Right? That would still save me having to import Retuner's settings, save, exit and reload everytime I start a new world for testing, which is what I'm trying to achieve here.

If I make substantial changes to the settings, I can just export a new tuning file, can't I? That seems simpler to me as I have to admit, you've got me a bit confused by your last message (sorry). I have also corrected those two lines for the Chicken Coop in my in-game settings as well so they won't be carried forward if I do export them again.

Rest assured though, I don't intend to do another 100 changes but you never know lol!

Gulp.... well that would be more work for you to find duplicates ^_^

With the ChickenCoop ITUN I was referencing to the adjust file I made based on your file, where I took them out of your Retuner Tuning File. It would be a simple copy and paste of that in my post to the adjusted Retuner Tuning File I did.
Since you already did it on the export (removed the <AgeSpecies>None</AgeSpecies> entry) and put the whole thing back in a new (?) Retuner Tuning File or within it, the reply I made about it was kind of void, but the instructions remains the same. You can just copy and paste "new values" (export file) on your Retuner Tuning File, just check for duplicates (*) before doing that and put them in the right sections.

As to loading the values in a new world, yes the Retuner Tuning File will load the settings in there, once you load up a new world, they will reflect in the in-game settings in Retuner as defaults.

(*) The reason why I say duplicates, is that if you are missing an ITUN behavior and decided in-game to put Autonomous on True, it might have been that you turned it on False that's already in the Retuner Tuning File. It wouldn't necessary break something when you play it that way.
But once you make a new export file and put it in the Retuner Tuning File, it may cause a conflict due to you adding something that's already is in there.

So by example:
<ITUN>
<ObjectName>Sims3.Store.Objects.ChickenCoopClassic</ObjectName>
<InteractionName>Sims3.Store.Objects.ChickenCoopClassic+FightChicken+Definition</InteractionName>
<Autonomous>False</Autonomous>
<UserDirected>False</UserDirected>
</ITUN>

and somewhere lower in the tuning file:

<ITUN>
<ObjectName>Sims3.Store.Objects.ChickenCoopClassic</ObjectName>
<InteractionName>Sims3.Store.Objects.ChickenCoopClassic+FightChicken+Definition</InteractionName>
<Autonomous>True/Autonomous>
<UserDirected>False</UserDirected>
</ITUN>

They will both be searching for "Sims3.Store.Objects.ChickenCoopClassic+FightChicken+Definition" where one value for Autonomous is "False" and the other is "True", that may cause a conflict in the Tuning.

Doing it with the Retuner Tuning File won't show you which settings are altered and the chance of you adjusting something that you already did (in the Tuning File) is pretty common. As long you separate whats in the Tuning File and whats in the in-game settings (which takes priority) there is nothing amiss with it.
But once you decide to export (again) and merge those values within the existing Tuning File it might error out.

Hope I've explained it more clearer now

Yes, thank you J4Ks! So really, all I need to look out for are duplicates, yes? I think I can manage that. Thanks again!