What do I roll to cast a summon with pre-determined base attack bonus?

I am playing a warlock in my current campaign and I selected earthen grasp for one of the invocations. Earthen grasp comes with its own base attack bonus:

Earthen Grasp:

Treat the arm as a Medium creature, with a base attack bonus equal to your caster level and a Strength of 14 +2 per three caster levels (16 at 3rd level, 18 at 6th level, and so on).

Looking at this, it doesn’t seem to require any sort of roll since the strength bonus is always pre-determined. In that case, what would I need to roll in order to summon/cast an earthen grasp?

Can Damage Multiplier and Predetermined Damage be combined?

Note: This question is based on something I saw built with the Dominus Exxet Ki Techniques, not the core book Ki Techniques, but I believe that there’s no difference at the level this question gets into.

Someone I know recently claimed that Damage Multiplier and Predetermined Damage stack, e.g. that if you have Predetermined Damage 250, and Damage Multiplier 3x, that the Base Damage of your Ki Technique is now 750.

This seems patently absurd, but the rules, as usual, don’t seem to be clear either way.

Is this the case?

Answers should preferably lean as close to rules as written as possible, though subjective answers aren’t off the table.