Fac air rifles and slug ballistic performance

[Aggy]

Hi guys I am now in a position to make slugs in both dish base and hollow base 
Both my fac air guns have I believe 1 in 17.7 barrels one is polygonal the other a Fx stx superior There will be an optimal slug weight and speed at which you can push it for those barrels my plan was to start with 20 grain and go up in one grain increments until I find what works with both slug types ,in both guns 
Is there a formula to use to get me in the ball park ?

I would assume the light 20 grain slugs will not be great as they will be supersonic unless I also turn down both guns to suit .

has anyone dabbled in fac with slugs