how much 300 nanometer ultraviolet radiation does plexiglass absorb?

There are too many kinds of plexiglass to answer.

All these non-glass windows are generally made of polycarbonate.
Plexiglass is polycarbonate, trademark owned by Rohm.
Lexan is another polycarbonate, trademark owned by GE Plastics.

Within the Lexan product line, there are tons of varieties.  Not just
thickness, but the polycarbonate made for outdoor use has a lot of UV
absorbers and UV preserving agents, anti-scratch agents, strengthening
agents, anti shattering agents.

Plain polycarbonate will transmit a fair amount of 300nm. 
Polycarbonate with UV dye will probably absorb 99+%.  That's a problem
I personally have dealt with a lot working with UV cured resins.

If you settle on a specific brand/variety plastic, contact the
manufacturer and I'm sure they will send you a transmission curve.

It is not quite accurate that Plexiglass is polycarbonate.  Plexiglass
is a common name for PMMA.