I'd rather go by
this picture taken at the NH kickoff that shows a field within tolerances. Last year we designed for tight field tolerances and got bitten by it in Atlanta when the fields sunk some due to weight on the layers of softer materials* which caused us to regularly get stuck in one spot. Never again will we design for tight field tolerances, ever. 17.5" or less, ftw.
*('regolith' on top of carpet on top of rubber on top of hard floor)