I seem to recall for a plate of glass (starting with the simpler case; I'm not missing Dave's point), the focal point is shifted back by t/n when a plate of glass is inserted between a lens and the film, t being the thickness and n being index of refraction.
I don't have a number for acrylic but I recall optical (eyeglass) polycarbonate having an 'index' (I'll swagger as I shorten 'I of R' to just 'index', and act like I'm
in optics as a profession) right in the mid-range that some optical glass does. 1.5-1.6 roughly.
My Crown Graphic (Ektalite?) Fresnel looks pretty thick, maybe 1/10 inch. Ignoring the lens issue for a moment, 0.1/1.5 is 66.7 thousandths, enough to really botch your focus.
For a cheapo sheet magnifier, ball park thickness 0.015", focus shift = 0.01". I forget the tolerance on 4x5 ...+/- 0.007"? but 8x10 and 11x14 might be +/- 0.010-0.015"
I have used cheapo sheet magnifiers & hoped for the best with brightening the image, despite unknown magnification.
So Dave, does the Fresnel factor alter the index-dependent focal point shift, or are you referring to an optimally selected one with magnification or f.l matched to the lens?