Not sure what year your 300 is, but if you have the smart craft gauges with digital read out, you can zero out the fuel used on the port and starboard gauges every time you tank up. Then you can see (approximately) how much fuel you burn. This is a good guess as to how much you'll need in each tank when you hit the fuel dock.
Good call, I'll try this next time I tank up.
BTW, any of you guys with the Smartcraft gauges install the vessel view mobile?