Have a look at Earned Value Management (EVM). There you compare your currently Earned Value (EV) (usually the cost that were planned to complete a specific task) with the actual cost (AC) for the task or the cost planned (PV for planned value) to be achieved to a specific point in time. Having those three numbers, you get performance indices:
Cost Performance Index CPI: EV / AC
Schedule Performance Index SPI: EV / PV
Your 15/10 formula provides the same message like the SPI. Due to the wide distribution of EVM, I recommend the 15/10 or SPI representation.
By the way, when you look for EVM, you'll discover that you get a lot more nice statistic values out of the 3 types of cost...