Monitoring Printers with Nagios

I'm using nagios to monitor my network and i want to add the network printers to it. They are mostly HP 2600N's. I've manged to find the OID for the maximum amount of toner, and the OID for the current amount of toner remaining. This is a little annoying as the maximum (M) is 470, the current (C) is 38. These values dont really imply much. I would prefer to do a quick calculation i.e. (C/M)*100 to get the percentage remaining. Any body got any ideas on how to do the calculations?

Thanks in advance