5th point harness slot in seat cushion

My wife and I are covering our Van's cushion seats ourselves. The fifth point on the harness clearly needs to go through the bottom cushion. However, since the seat back (and bottom) may be moved fore and aft anywhere within a range of about 3 inches, how do you decide where, or how big to make the slot? Do you make it for the middle position and then just allow the 5th point to pull at an angle through the slot when the seat back is adjusted farther forward or aft? Will it go through OK like this? For those who have purchased their finished seats from a professional, how did that professional deal with that?
Thanks for any suggestions.

I just did mine, had the seat back in center position and cut it the width of the 5pt buckle and back about an inch. Just so the buckle can be removed from the seat bottom.