Well, it depends on which Pactra you're referring to, there is Pactra Acryl which I know NCNitro uses with great success. I personally prefer to use Pactra Racing Finish. Acryl is water based so you can thin it with water as you could with Parma's Fascolor. However, Pactra Racing Finish is Laquer based so you need to use a laquer thinner. If that's what you're intending to use, allow me to give you some advice... DO NOT use any type of laquer thinner other than the thinner made by Pactra. I know you can get a gallon of laquer thinner at Lowe's for the price of a bottle of the Pactra stuff, but the Pactra thinner or formulated for their paint and works MUCH MUCH better. I only use the cheap stuff for cleaning.
If you are going the water based route you can simply use water. I've also read many times that people use a little bit of glass cleaner (Windex) to thin the paint. Not much... like a tablespoon per bottle or something like that. I never tried it because I switched from water-based to laquer-based before I heard about that.