Problems with using the POH might include
- a different prop has been fitted
- prop overhauls, removing metal (overhauling a fixed prop means a higher RPM is needed for the same thrust)
- the engine might be knackered (the fatal IOW one last year had so much metal missing off the camshaft that the valve lift was reduced by 40%)
- the engine control end stops might be set wrongly
- the RPM gauge might be off (power is proportional to RPM cubed, I think, so a 5% error means a 15% error in setting the power (=thrust) figure)
- the 30 year old POH might have been hacked around, bits replaced, etc.
etc.
500+ miles to dry, 400 with sensible reserves
is not good! What if the destination is closed due to say fog? You have to fly say half an hour to an alternate, and then you have almost nothing in the tanks. And fog tends to be widespread. A 1hr reserve is not enough. With my 1% accurate flowmeter I can do this (but wouldn't).