My level line was off by a half inch on a 30 foot garage wall

Last Tuesday I was laying block on a garage addition in Tacoma. I had my line pulled tight, checked it twice, felt good about it. Got about 12 courses up before the owner came out and said the door opening looked crooked. Sure enough, my line had sagged right in the middle and I never caught it. I used a laser level to verify and I was off about a half inch over 30 feet. Had to tear down and redo two hours of work. The problem was I used a cheap nylon line that stretches in the sun. Switched to a braided polyester line and now I check the middle with a straight edge every 5 courses. Anyone else had a line sag issue that cost them time, and what do you use to keep it true over long spans?