John - excellent suggestion re. grommets, but the groundsheet need to be smaller all around then the tent floor -- with extensions coming out at each corner, and with a grommet at each end.
Considering that polycryo sheets will acquire pinholes and tears eventually, I don't think it's worth the time and effort cutting out one in precise dimensions, taping, adding grommets, etc.
If I really want a groundsheet for fast fly setup, etc. -- I would buy a dedicated one -- knowing the dimensions will be accurate and the grommets positioned just right -- and heck, even the fast fly buckles will already be there!
And if I really want to cut weight, I would cut out the groundsheet, leaving only 1-2 inch wide nylon strips connecting between each grommet (picture a skeletal "Union Jack"). I will then apply short strips of velcro -- one at each corner plus one at the X perhaps -- then velcro a sheet of polycyro on top and cut away excess material.
The polycryo sheet can now be separated easily -- and a new one applied when needed. Just keep a small supply of velcro strips handy (for the polycyro side).
EDIT: It's Polycryo by the way.