For the bug smasher, determine the fuel flow in pounds per hour (X-Plane does everything in pounds). Do that by telling X-Plane to display fuel flow with the plane at your "usual" cruise altitude, cruise speed, leaning appropriately, and seeing what the fuel flow is. Divide by trip distance to get fuel needed. Add legally required reserve fuel (30 minutes if VFR day, 45 minutes otherwise). Add takeoff/climb fuel. Add "Oh oh" fuel.
A crude estimate of climb fuel can be made by noting the fuel flow at takeoff, noting the average climb rate you get during climb (e.g., if you get 1500 fpm at takeoff from sea-level but only 700 fpm at 8,000 feet, then figure an average climb rate of 1,100 fpm. So, climbing to 8000 feet from sea level will take you roughly 7.5 minutes; multiply by your takeoff fuel flow).
Or you can get more sophisticated and do the flight testing to make a time/distance/fuel-to-altitude chart
. It only takes an afternoon.
"Oh oh" fuel is fuel for weather diversions, holds, & etc., so that these things do not eat into your lawful reserve fuel. How much "Oh oh" fuel you add depends upon the nearness of available alternate airports, the badness of the weather, etc. Flying in Socal on a good weather day? You might not feel the need for much, with plenty of airports around. Flying in Alaska where the airports are far apart and the weather changes quickly? Fill the tanks to the brim!