Is there a chart or formula for figuring out what oil weight to piston hole diameter?

I'm asking this because my track surface temp is about 55 degrees. Using typical setups, I get too much pack in the shocks. I was running maifield's setup in higher temps and the truck was performing great. To reduce pack, I have dropped down to 25 wt. front and rear, but would like to go lower in the rear. However, since I have removed the upper x-ring to reduce friction (this made a bigger diff. than I thought it would and don't want to go back to two x-rings) I am worried about blowing the thin oil out as if I stayed with the balance recommended, I should be running 17.5 wt. rear shock oil. If I went with the 1.3mm pistons instead of the 1.2's, about how much higher in weight oil should I go to get the same feel? (Using given setup at a constant temp) I will go my two to three weights lighter for my track temp.

I understand that it won't be exactly the same feel due to fluid dynamics.