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)

For example if I was to print out a cube with 50% infill, would those infilled layers have 50% of the cross section area compared to the fully solid base layer?

What I'm doing is trying to print out a sort of net/ mesh type object. I'd like to be able to calculate the % of the cross sectional area that a fluid would be able to flow through.

Thanks in advance!