How do you fairly meter propane use for duplex?

[Peter B.]

So I am looking at this duplex and it has propane tanks for each unit. So my question is what is the best way to meter the usage between tenants? Take a reading and give vacating tenant a credit? Then apply that additional amount to the incoming tenants move in cost????.. That's what I came up with does anyone else have a less cumbersome way of doing this? 

Thanks, Peter