You can configure the profile for your aircraft however you like, including modifying the capacities of any tanks. As SkyDemon would have told you when you selected the R44 profile, you _must_ check it to ensure it reflects your actual airframe.

The slider on "Flight Details" for a R44 helicopter goes up to 49 US gals.  Is it possible to enter a specific amount as a full fuel state?

For background, R44 tanks historically held 48.9 US gals, however after the Robinson AD to retrofit all existing tanks with bladders the present full state is 46.5 US gals.  I'd like set the slider to max at the full fuel amount.

Would probably a good safety check of the software as someone could possibly slide to max (49gals) thinking he's going with full fuel, but be off by few gallons in practice.