I've got an old 505 that my dad purchased back in the 80's, and a Lee Safety Scale of unknown vintage (old) that was given to me by a cousin. Both of them measure to within a 1/10th grain of each other (probably less than 1/10 actually). Without question the Lee is a LOT more clunky, and less dampened, but it is accurate.
The Lee should be accurate, and if you find it isn't accurate I would call Lee and try to figure out where the problem is because it should work just fine. The 505 is a nicer working scale however and will make for a nice upgrade. The only problem I ever had with mine is that it sat in dad's garage for about 10 years unused and it got a teeny tiny spec or two of rust on the fulcrum. Nearly pulled hair out on that one until I looked really close under some good light.
Some suggestions for either a Lee or a 505:
1) Don't use when the heat, AC, or fan is running. I don't know whether it is air movement or change in temp, but whatever it is the things don't work as well.
2) Zero it, and make absolute sure you don't move it or bump it. I believe I've seen some pictures of the setup Dryflash (??) uses as a solid mount for the scale that seems like a great idea, but really as long as you are REALLY careful not to move it you should be OK.