This is my procedure when testing fuel usage for a new plane / engine combo.

Set you timer to something safe, say 5 minutes. I like to use the Futaba function where the timer is 'on' only when the throttle stick is above a programmed percentage. I usually use slightly above idle. (Not sure if you can do it in JR).

Flight your typical style..

After landing, if you can't see the fuel level, take out all the remaining fuel into a separate vessel. I 50oz dubro tank works nicely. Take note how much was left. If enough, add 1 or 1.5 minutes to your timer. Keep doing it, until you only have enough to make you fell comfortable as reserve. It may take a few flights, but should get you there safely.