I'm using a 'store-bought' jag labeled for a .32 in my .36 barrel, and like it. It allows the cleaning patch to be pushed to the breech without pushing too much fouling down the bore ahead of it, then grips the patch and bunches it up to pull a lot out towards the barrel. I'm also using a labeled .36 jag in my .40. So, I'd say that .040 to .060 under size works for me and the cleaning patching I'm using.
Lately, I haven't been experiencing the vacuum sucking the cleaning patch off the jag and back down the bore like I used to with a tighter patch/jag combo, either.
Like Dennis said, whatever feels right to you.