For a hw97 mk3 in .177

26mm setup
Piston weight is 210 grams (which I realise is on the light weight side for a 26mm piston, but when it works recoil is low)
Stroke with O ring piston head 77mm

I'm going to tap the present port to take M5 grubs screws and re drill. Can anyone give me a ballpark where I should be at for this configuration?

I expect I'll be using a fairly long soft spring rather than short/stiff, but I have options.

I've had the above working quite well with a 3.9mm port, but I'm not convinced its correct as it was feeling quite slammy which is why I'm going to re-drill smaller.

Thanks!