How to judge if the power of laser tube become weak

Please find a piece of 6mm acrylic (50*50mm),which used for testing the power of laser tube.

Like below.

Files: cut a 15*15mm Circle ; blowing air; keep laser power to 70%,and then test how much is the fastest speed does the laser cut through the acrylic.

Please tell us your testing and we will judge for you.