How powerful micro-photon torpedoes are? I wouldn't say more than 1-2 megatons, if we scale them, and HTL are about 1.5 MT (ANH novelization, "vaporize small town" issue - link here.).
@Jason
Yes, it does tell us. We have RotJ space battle, and possibility of some quite iffy estimates from that - anyway, I wouldn't give more than 250 MT total for ISD's shields (link).