If you still have the original gauge, connect it temporarily
to the sender and measure the current with an ampere meter.
Once you know the current flowing with the original gauge, then just
dimension the resistors accordingly.

Old thermally-actuated gauges use a voltage stabilizer or "vibrator" in
order to compensate the voltage changes when the generator is charging.
In this type of gauge the current will not constantly flow through the
rheostat of the sender. See URL.

http://www.nls.net/mp/volks/htm/fuel_ga.htm

Another type of gauge is the balance coil gauge.
No voltage regulation is required with this type of gauge.
In this type of gauge the current will constantly flow through
the rheostat of the sender. See above URL.

* * *

Your circuit is fine because the voltage is regulated.

* * *

When you go around a turn, or up a hill, the fuel can slosh
to one side of the tank and quickly change the float position.
If the gauge needles were to respond quickly to all of these changes,
it would be bouncing all over the place.
The capacitor C1 on my BMP was to limit this problem but you
can also take care of that in the software.

If your fuel sender is a wirewound rheostat, then don't expect
a resolution better than E, 1/4, 1/2, 3/4, F.

Example of modern fuel sender on ceramic substrate.
http://www.bourns.com/pdfs/fuel_card_apno.pdf

Again, be careful!

Luciano