KOHLER KDI Cylinder Head Flatness Check

A 2x4 is not a straight edge tool. Neither is a glass window or countertop, or a bench rule, or a yard stick, or any number of other creative things I've heard of being used.
A certified straight edge will have documented tolerances (how straight it is over a given distance). A really good straight edge will indicate a temperature range in which it will remain within the listed tolerance. It will be made of granite or some type of machined ferrous metal.
The cylinder head must be free of debris and gasket material before measuring flatness.
If out of spec, which is a rare condition for the cast iron head like the one shown in this video, odds are something caused it to loose flatness (warp). The most common cause is excess heat from overheating the engine. Be sure to find and fix the cause before replacing expensive components.