As belial said, depending on how overt this measuring system is, it could be very disadvantageous.  If there is a lot of wizard duals or wars going on, an entire science could be made into creating devices that are hidden or provide incorrect information to others that could see it.  Do they have to be calibrated to specific wizards, or can anyone use them?

That being said i think your big question you need to answer is; WHY did these people come up with a way to determine how much power, exactly, they have left?  From what you have described already, it sounds like each individual wizard has a way already of at least eyeballing how much power they have, as the more they use, the more tired they get and at sever enough levels begin suffering obvious physical injuries.

Have you thought about making it so they couldn't determine how much power they have?  Perhaps as they use their power, it causes a natural high, either endorphins or just raw energy being converted from fat/muscle which makes them feel good.  This could prevent a wizard from knowing just how close he is to causing himself physical harm( no longer feel fatigue or drawing enough power just an immunity to pain), and would spark a need for some way to keep track of your power, especially for newbie wizards who are not yet accustomed to their limitations yet.