: 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.

Good point. I always make sure that my boards bow up in the center when I set them up, so they introduce extra rocker, but I select boards that show as little curve as possible. It's better to run a stringline under the station blocks to check this; I used to work as a carpenter and I've seen some pretty uneven floors, especially in garages with drains.

****
Another approach is to simply ensure that your strongback will remain stable, but not worry about the 'perfectly straight' issue. Using a string/wire and level/plumb bob to set up the forms, it's easy to work with a less-than-perfect strongback.
Also, the 'trick' of lightly spray-painting over a stretched string to establish the center line on the strongback is a very good idea- I can't remember who to give the credit to for that one- another idea harvested from online.