Just wondering. Or is it just a flat amount for the shield?
Power Word: Shield has a 10% spell coefficient in Classic according to Wowhead Classic.
So if you had a piece of +10 spellpower gear you’d get +1 spellpower to your PW:S. Same with +healing power afaik.
Source:
https://classic.wowhead.com/guides/formulas-spell-coefficients-classic-wow#introduction