The correct way is to check game database (training -> player collection -> find your player -> press and hold) for S99 skill and S1 skill power/momentum. You can also use websites like Tsubasa.im to do this.
So for example, if S1 is 100 power and S99 is 250 momentum, it means that 100 levels is 150 momentum so each level is 1.5 momentum (approx).
Let us consider another example, if S1 is 200 momentum and S99 is 400 momentum, it means that 100 levels is 200 momentum, so each level is 2 momentum.
So for the first example, one SSR BB is worth 1.5 momentum, while in the second it would be worth 2 momentum.
For A skills, the highest level is 80 so for example, if A1 is 100 momentum and A80 is 140 momentum, it means that 80 levels is 40 momentum, so each level is 0.5 momentum.
As you can see, the amount of momentum per level depends on the (MAX - MIN) momentum divided by the number of levels for that skill. S99, A80, B60 and C40.
Each BB will give different momentum, depending on the skill.
Generally, that's why SSR BBs should only be used on highest momentum shot skills, as they give the most power per level.