I use the Bego bellaStar XL investment, I could control the casting fit easily by adding more or less distilled water as mentioned before, I have found, in order to have a consistent result to measure the thickness of wax coping befor investing (wax caliper) and invest the thin wall coping in separate ring(plastic ring)and the thick coping in another ring (plastic ring)to control the expansion separately, when I invest thin and thick wax coping togother in one ring the result is the thin wall metal coping fits good while the thick metal coping is tight and vise visa. I do not know if it makes sense with my fellow technicians, but it works for me, another thing you have to keep in mind is the setting expansion and the thermal expansion of the investment, I think there is a defference in the fit of the casting results between metal or plastic rings being used for investing ( expansion ) my investment high temp for ceramic alloys if 1560f, hoping this will help and I am open for any suggestion, thanks