I only use a little PGA (in my case actually 160 vodka), talking like 5%, 0.15ml in 3 ml. I use it primarily for thinning high VG liquid, don't think I've ever really noticed much TH change when it is used.
If you have a tobacco flavor that you want to bump up, try getting a little Tobacco Absolute diluted, from the perfumers apprentice. Just a couple drops per 10ml of liquid (no more than about 4 drops) really beefs up the flavoring. TA diluted is the stuff that they dilute with 50% PGA (much easier to work with).