So in another post recently about doing alignment on your own, a common thread through many responses was that you have to have a flat floor. So two questions

- how flat is flat?
- how do you check for flatness

I am curious what people consider 'flat' when doing alignments, and how they arrive at that conclusion. I have seen in cataloges nice granite inspection plates (6'x8&#39 which are +/- 0.001" throughout. But since something like that costs about $7500 and weighs 4 tons I doubt too many people are using them for IT car alingments. My shop floor is not like the paddock at Bridgehampton, but it isn't as flat as an inspection grade granite block, so I'm curious where it stands.

Thanks in advance.
Matt