Weight x Reps x .0333 + Weight = Estimated 1RM
From there take 85%. Jim's #1, numero uno, the FIRST principle: Start Too Light! Why blow your ass out for a 1RM if you're going to adjust down to 65/70% anyways? Rock on, good luck.
Edit: Sorry I should have read the comments before replying, it was already answered. Git er done.