Also, the conversion factor (70%) is also the (paid for $time/premium for $time) price ratio, so your method would have the same result as what currently exists. However, I'm not clear whether you also meant your suggestion to cover the "let me convert my remaining paid time to premium paid time without buying additional time" use case (eg, you have 100 days paid time left, and you convert them to 70 days premium paid, without buying additional premium paid time).