I do the same as you - only use engine hours and try and always keep tanks to the brim to reduce condensation.
I did have fuel guages and as I could not trust them I removed them. I will cruise at about 2000rpm which gives us 6.5kn in most conditions and I adjust for seastate if it is rough. We have the Yanmar 4JH2-UTE. Usually rough seas means some breeze so we sail in that case. I hate the ride through seas if only under motor.
The last 'larger' section I did, I used the motor more than I wanted and after 750nm, I needed 50L.
Just mucking around the Bay, I will have used more than this - especially after my father decided the turbo needs to be used more~3400rpm.
As for the running out of fuel, well, hopefully not and even though I am almost in suburbia now, I still keep another 4 20L jerry's
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