lifter pre-load?

Even the heavy duty paddle rockers are not very accurate as far as geometry. Also, look at the slop in the holes where the shaft goes through. "Heavy duty" or not, they're still just stamped formed steel rockers. They AIN'T real accurate. If the lifter preload bothers you that much, you should get adjustable rockers and be done with it. You can zero in on it much better then anyway.