ASPD = 200-(Delay*DelayDecrease)
Delay = (250-AGI-(DEX/4))*(200-ASPD Base)/250
DelayDecrease = 1-Speed Mod
This is from irowiki, ..here's how you calc it:
ASPD Base = 140 for bows. (if a hunter wields it)
So for example I have an archer with 90 AGI and 95 DEX total, and is using a concentration potion.
(250-90-(95/4))*(200-140)/250 = 32.7
Concentration pot = 10%, = a mod of 0.1
DelayDecrease = 1-0.1 = 0.9
ASPD = 200-(Delay*DelayDecrease) = 200-(32.7*0.9) = 170.57, which will be truncated to 170
If the sniper didn't use a concentration pot, or any other delaydecreasing items/gear, just do 200-(Delay)