yup, one layer would be 50% filled, 50% unfilled, spread evenly across the object.. and not including the perimeter walls.
however, depending on the style of fill layer you choose, you may end up with greater coverage than that, depending on how the settings are configured.
as an example, this is two layers of a basic 50mm cube, 3 perimeters, 50% rectilinear fill left at basic default settings in Slic3r. One layer gets laid down in one direction and the next layer goes down perpendicular to the first, effectively reducing the visible open area to only about 25% (+/-).
I am sure there are mathematical formulas that could give you the exact figure, but I have no desire to dig into it that far.. I am simply looking at it from a visual standpoint - ie: what I can see..
What you end up with will be heavily dependent on how you configure your fill layers, and how well calibrated your printer and filament are (obviously overextrusion will take up more than the desired amount of space, etc)
SD4 #1 & #2 - Lawsy carriages, E3D v6, Rumba controller board, mirror bed plate, X motor fan, upgraded PSU & Mica bed heater
SD4 #3 - in the works ~ Folgertech FT-5, rev 1
Printit Industries Beta Tester - Horizon H1
