We use Lowrance but only because we have Lowrance network already on boat. All the device does is measure fuel flow, but this value is then used to calculate a load of other stuff. The figure you want bieng one of them. Ours was quite cheap about £50 I think.
There are some limitations with the Lowrance device, so far we have not found a way of removing fuel, you can add it but not remove it, it also does not seem to be able to deplete one tank and go to another (it sees both tanks and subtracts equal amounts of fuel from each) it could be the numbnut trying to set it all up though...
New boat is here, very happy!