Welcome ExiledFish!
If you're not against using mods I would suggest taking a look at the
Keyboard Function Mod 1.2 by CCIP. It's for SH4 and adds some additional funtions that are useful for managing fuel, most notably Shift+G which will tell you your maximum range at your current speed.
This is also available in SH3 by clicking on your Nav Officer, Reports and then Max Range at Current Speed.
Basically, the Maximum Range at Current Speed report is essential when playing with limited fuel in my opinion.