QUOTE(The Wiki)
Spirit Damage = Base_SP * SS_AP_Damage * MIN( (Damage - Max_HP * SS_AP_Damage) / (Max_HP * SS_AP_Damage * 3) , 1)
(SS_AP_Damage = percentage of damage required for Spirit shield to kick in)
QUOTE(BlueWaterSplash @ Sep 23 2018, 23:42)

In the typical case the formula reduces to: Spirit Damage = Base_SP * absorbed HP damage / (Max_HP * 3)
Note in particular that the more Max HP you have, the less spirit damage you take. Since HP increases linearly with your level, Spirit Shield uses much less spirit on high level players. It's also important to get lots of juggernaut to make Spirit Shield better.
The problem with Spirit Shield on low level players is that it uses too much SP. Training does not reduce the SP damage.
Wait, I have to reconsider Juggernaut's interaction with Spirit Shield once again. Not only does Juggernaut make Spirit Shield less trigger happy and save SP there, it also directly saves
way more SP because the Spirit Shield drain formula itself is inversely proportional to Max HP.
Perhaps Jug25 is an absolute must after all for anyone who uses Spirit Shield.
You could even say that while Juggernaut HP is not "real" since it doesn't help Cure or your MP situation, it is "real" HP if you imagine Spirit Shield to be like healing that uses SP instead of MP.
Although, the Spirit Shield formula references Max_HP not just the bonus Juggernaut HP. At level 500 the SP drain on 1H may be so little even without Juggernaut that you don't care. The Regen spell also uses less MP at high level (because it lasts longer) but Cure and Draughts are roughly the same at all levels.