Heifer weight for breeding

What would be an appropriate guideline for a heifer to be bred?

I generally thought the rule of thumb was when she is 2/3 of her mature weight. So if your mature weight is 1200lbs, you would be safe to breed her when she weights 800lbs. In practice, I run all heifers with the bull when they are ~15 months of age, so most of them calve around the 2yr target, regardless of the what they actually weight at yearling+.