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In D&D 5, the saving throws for spells has the following formula:

The DC is set to 8 + prof bonus + magical stat bonus.

The target then makes a saving throw, adding their stat bonus and hoping to get above that DC.

Instead of having the defender roll the saving throw, we would like the attacker to roll the attack against the "defense" of the monster. (Mainly to make play by post run smoother)

Would this mean that the attacker just rolls d20+prof + magic stat bonus, and they try to get a number which is higher than 8+ monster def stat bonus, or is there a different formula which better maintains the math? (This math sounds wrong to me, so I assume there is a better formula)