I wouldn't trust a measurement to the floor as many floors are not as flat/level as one would assume.
Steel is real, as they say. This fixture has been used on a dozen boats so far, it is 3" square .120" wall thickness.
Why deal with potential warping, shrinkage or just the sheer mass of one of those giant wooden strongbacks?

I use this and 2 string lines thru the forms, at the top and at the bottom of each station. This checks for twist as well as just straightness.
The lines ate kept taught with a spring. All the adjustments you could want, but Wee Lassie forms (we have built 3) do need to be spaced up a couple of inches so the stem forms clear the tube.
I work inside (often the living room, thanks to a very tolerant wife) but do all the sanding outside, thanks to the "landing gear" which fits thru a 36" entry door.
The whole thing comes apart in minutes as everything is bolted together.

I still can't figure out how to get my pics to show up in a post, so it's here http://www.flickr.com/photos/17079978@N08/6480373747/

: If you are measuring from the top of the spacers down to the
: strongback, make sure that there is no sag in the middle of the
: strongback. Before you set up your spacers, measure from the
: floor to the strongback at both ends and the middle to ensure
: that it is perfectly straight. There is no point setting the
: riser heights to within a mm, but have a 10 mm sag in the middle
: of the strongback.

: John