This is one of my fav "lol" at it regs. We devolved reduction in windshield and frame into, well, not a windshield and frame.
Open our favorite rulebook to 9.1.5.E.9.a.8.A, page 483 in the May GCR.
A allows removal of all glass and frames. If removed a replacement must be added. That "replacement windshield" must fit "within the vertical planes of the front most and rear most elements of the stock windshield and frame. The replacement windshield must not exceed the height or width of the stock windshield and frame. Any portion of the windshield that is in the driver’s line of sight, must be constructed of a clear material."
So make a triangle that consists of the side view of original windshield frame and as long as the "replacement windshield", which must be made of polycarbonate, is within that triangle, you're free to do anything you want. No farther forward than the base of the original windshield frame, no further back than the top of the frame, and no farther laterally than the side of the frame.
Naturally, for aero purposes you'll want to make it as small as possible (with appropriate minimal cage elements exposed to drag).
We call these "Ginther Windscreens" because Richie Ginther exploited this allowance on 914s. Now everyone with an open top car is doing it.
This GT Racing high quality light weight Richie Ginther windscreen is intended for the track enthusiast who wants to add quality aftermarket auto parts to their Porsche 914
www.gt-racing.com
And as for that rear glass on targa cars? Allowed to be removed entirely if configured as an open top car (paragraph D). Which of course you do for aero advantage. T-top cars cannot take advantage of this reg (I tried to get it approved on the Toyota MR2, denied).