Can always check the ingame Help menu for things like this, but in essence many buffs/debuffs work on a percentage basis. So if you have 1,500 spell damage Minor/Major sorcery (Improves spell damage by 5%/20%) would be more effective than if you had only 1,000 spell damage.
A list of all buffs and debuffs can be found here, but the in game help menu also has this information.
https://en.uesp.net/wiki/Online:Buffs
I think shield is based on magic or stamina for the bone shield, however its capped at 40% health.If its said its scaled on maximum health, it is nonsense to be scaled by max stamina too.
Only your health can increase shield size of Bone shield, even when it costs stamina. Stamina there is just price for using this skill