http://sixriver.web.fc2.com/ssb4/Guard_Hitstop.htm
Apparently sixriver has a shieldstun calculator now, which calculates attacker hitlag, shielder hitlag, and shielder shieldstun, and has formulae for finding these three things. It also calculates frame (dis)advantage for using a move on shield. I have no idea how accurate it is, but it seems to match up fairly well with what I've observed.
The supposed formulae are:
Attacker Hitlag
(Attack Base Damage ÷ 2.6 + 5) x Hitlag modifier x Element modifier [1.5 if electric, 1 otherwise]
Defender Hitlag
(Attack Base Damage ÷ 2.6 + 5)
Defender Shieldstun
Attack Base Damage ÷ 2.56
A translated calc example using
usmash:
Remember to add 7 frames of advantage if they need to drop their shield to punish!