On the surface (pun intended), the fill lines on the tank seem self-explanatory. And they would be indeed, IF the resin lay perfectly flat to the edge of the tank. But in my experience, the surface tension of the resin causes it to creep up the sides like so:
Assuming the fill lines refer to the level of the resin throughout the tank, this phenomenon makes that very hard to judge. Could it also be fooling the Resin Sensor in this way, thus being a factor in the “Resin Sense Error” that’s popping up in the user community?
Hopefully the surface tension isn’t quite that pronounced though I quite like your tank model Judging the fill line from a front view will be a good approximation and it shouldn’t be necessary to compensate for the surface tension.
In my experience, the resin fill doesn’t need to be all that exact. A tiny bit over or under won’t be a problem (actually, a lot under won’t be a problem).
As long as you’re “about” on the line, IMO there’s no need to worry.
Judging the fill line from a front view will be a good approximation and it shouldn’t be necessary to compensate for the surface tension.