Hi,I'm trying hard to understand all of this.
In my mind I can use unusable fuel feature if the empty weight does not incluse it,
And wont use it if it is still included in empty weight and CoG data.
Btw the key is "what does the manual book say " ? Include or not ?
If included, the gauge or the stick should start at included = differential zero
If not included, the gauge or stick should start at zero = absolut zero and you use unusable fuel feature.
Of course if you say 10L included in empty weight but , but gauge indicate 10L at unusable, and the CoG does not include it and so on, then you cannot solve the problem because your data are uncorrelated.
What I mean is that your data must be correct/coherent with the first point -unusable fuel is included or not in data book- and then the other data depends on that first point.
So far I dont see something wrong in SkyDemon.
(No need of a tick box ?)
HTH.