As far as I'm concerned, by introducing the power of a laser in E=mc^2, I get the amount of antimatter produced (actually the amount is half of it).

But in order to produce antimatter, a laser has to be fired into electrons, and I assume that the higher the power of the electron-beam generator, the more antimatter is produced.

My question is: how can I calculate the amount of electrons that needs to be produced (with the electron beam generator) in order to create a certain amount of antimatter?

Thank you in advance.