I'm primarily WHM, dabbling in SCH from time to time... So I might be inaccurate on the matter, but from my experience and the general consensus in stat weight, I'd say amount healed... Not HP recieved is the deciding factor. So in your example, the player would be shielded for 500 damage, not the 250 the player was healed for. 1500 were the spell to crit for 750.