How Flat is Flat Enough?
"Flat" is a VERY Relative Term
How flat is flat enough when it comes to working with hand planes? My rule of thumb is, "if you have to ask, it is flat enough". Put another way, if you can close a joint or press two faces together with hand pressure, it is flat enough.
The biggest issue is when a woodworker starts chasing a level of flatness that not only isn't required, but is only achievable for a short period of time until the wood moves with changing humidity. Or just exposing free wood will cause the board to move out of flat. So I find it is best to always step back and ask myself how the surface I am working on will reference against another...if at all. In this way, flatness can only be determined in the context of a project and there are many factors that influence that determination.
Flat is Flat Enough When...
Learn More in The Hand Tool School
Flat is best understood in the context of a project. I've got lots of projects I have build with comprehensive discussion of flatness and design already in The Hand Tool School. If you want to up your hand tool game, this is the place to be.
So I have a deal: use "rwwlive" to save 10% at The Hand Tool School