If you are using 1/2" pex, I'd use a bigger hole! The OD of 1/2" pex is 5/8", and I can tell you from experience, you'll quickly find it very difficult to pull pipe through. For that size hole, I'm not sure it makes that big a difference along the length of the joists. if it was a bigger hole, then I'd be worried. Wait for one of the pros.