Flatness...
 

Hello,
What is the best and most accurate way to check that your building surface is "flat"? I don't necessarily care if it is "level", you could build a whole plane on a surface that is way out of level but perfectly flat. Is it as simple as taking a 4' level and checking all different positions and making sure your surface is in contact along the entire length of the level (not caring where the bubble is)? I would tend to think so but would like to hear from you. Thank you.

Justrfb (checking all my options)

Flatness...
 

Werks for me. :)