Hi, calculations are one thing, real life another. I have belt driven machine on all axis. So i believe i know something about belts. The only thing positive in this design is that the belt is perpendicular to the ball screw, means its not driving directly the machine but transferring the movement.

What size type belt and pulleys?

How heavy is the gantry with all motors, rails and so? Not sure if you could drive it with one 300w servo at all.

I know Dean /JazzCNC/ is driving his machine like this, but you have to make sure about the proper belt size, corresponding motor and make sure that the accumulated inertia of the pulleys and ball screws will not limit the machine speed.

And looking at 0.05mm means tthat you must strive at 0.01mm. That means cheap chinese pulleys will NOT work. Have that in mind. And if you need to lower the inertia you must buy expensive aluminum pulleys or custom made that small size. I have 20 crap pulleys laying around cause i thought they were a deal from aliexpress.