I wasnt sure weather to do it this way or take the 4 Gallons right at the beginning from 22, then work on 18 to get CofG and Fuel consump
Right second time around.
The basic empty weight, and the aircraft's moment on the w+b schedule, includes unusable fuel. So, if the total fuel you're starting with includes unusable fuel as per your example, then you need to subtract this from your calculations before starting to avoid double-counting it. It is only the usable fuel which you need to include in your w+b calculations.
In your example, you need to clarify exactly what you mean by 3/4 tanks. Do you mean your tanks contain 3/4 of the total fuel? Or 3/4 of the useable fuel.
If 3/4 of the total fuel, then the fuel carried is 16.5 gallons. Subtract the 4 gallons of unusable fuel to get the usable fuel carried (12.5 gallons), then continue as per your description (but no need to subtract unusable fuel again).
If 3/4 of usable fuel, then the tanks can hold (22-4)=18 gallons usable fuel. The usable fuel carried is 3/4 of this, i.e. 13.5 gallons, now continue with this as per your description (but no need to subtract unusable fuel again).
Hope that answers it,
FFF
---------------