Why bother with a (assumed) rectangular tarp, that won't fit easily and tautly over the pole set? Just use the rain fly that came with it. Most tent manufacturers actually tell you that leaving the inner tent behind, and using a footprint-pole-and-fly setup is a great way to minimize weight.

By the way, this is only practical if you don't camp in bug country. Otherwise, you'll spend every minute of every night regretting the decision to leave that wonderful bug netting behind.