Is it possible to set an FL for a leg?  If not, how do I plan around airspace bases measure in FL?

Is this a feature on the roadmap?  If not, is there a way to request such things?

I’ve no idea, I’m just a user like you, but there is a suggestions section on this forum.

My question would be why do you need it to be so accurate? Once you go down this rabbit hole there are all sorts of questions raised that would have to answered.

Ultimately what matters is setting your altimeter correctly according to the level you are flying and airspace you are in or under, and flying accordingly.

In flight, Skydemon itself can only depict your level according to the best information it has, which is almost certainly going to be a GPS geometric altitude unless the device it is running on is being fed a 3D position by something quite sophisticated. Your vertical position will often be depicted differently to the magenta line even if you are flying exactly at your planned barometric level. You just have to accept that.