Mark, you are correct - Payload is useful load minus fuel. In some cases with the jets I fly they use BEW - which is empty weight + crew - then the calculations for fuel / payload are then computed.
For a SR22 for example (lets use a G2) it has a typical useful load of 1050 lbs. It can hold 80 gallons (nominally) so the payload with fuel fuel would be : 1050 - 480 = 570. If for example you wanted to fly for 1.5 hours with a fuel burn of 17 GPH , and VFR reserve of 30 minutes your payload would be: 1050 lbs - fuel (2.0 x 17 = 34 gallons = 204 lbs) = 844 lbs. A C182 would be very similiar - just a lot slower than the SR22 but faster than the Chinese plane.
I use the Joby, which is a great prototype, since it is one of the most popular in the electric aircraft race, albeit a different class.
I’ve flown the electric Pipestral in Europe and for 30 minute flights is it is a blast.