I tried all the methods suggested here, the scratched neg, using small levels and checking each stage but still had problems with slight unsharpness in part of the print. The only way that I could improve the sharpness was to stop down the enlarger which resulted in quite long exposures. Three years ago I did a printing workshop with Richard Newman for Calumet in the US and he introduced me to the Versalab Laser alignment tool which O promptly purchased and have never regretted it. I check the alignment every day that I'm working, it takes about two minutes and I know that the prints are goibng to be sharp from corner to corner. Save yourself a lot of frustration and wasted time and buy one, you'll never regret it.