I've read a lot of posts about subfloor flatness and I have no problem determing the size of a low spot using a long straight edge, but I just don't get how I can figure out the maximum acceptable height of a high spot. I'm planning to install wide plank flooring on a plywood subfloor. Most of the subfloor is no problem with slight variations from joist to joist. However, there is a section 3 joists wide (a ridge) that runs the width of the room and is higher than the surrounding subfloor. This is where a basement cinder block wall is located. If I place one end of a straightedge on the ridge and go out in any direction, there is a barely noticeable gap under the straightedge or none at all. If I center the straightedge on the hump, there are gaps at both ends. What is the acceptable limit for these gaps ands at what distance from the ridge? Do I average out the gaps a certain distance from both sides of the ridge?

Thank you

You want to be within 3/16" in 10'. You can always just sand the hump down. Better to do that now than create a problem later.

So if I have a 10ft straight edge centered on the ridge and have a maximum of 3/16" space at each end of the 10ft straight edge, am I ok? What long-term problems might I experience if I installed long, wide-plank flooring (Hemlock) spanning this ridge if the boards are nailed and glued (PL Premium) to follow the contour of the ridge (i.e. no hollow spots between the flooring and the sub-floor)?

Thanks

No, that would be 3/16" in 5', not 10'. So you would be over tolerance double.

Most likely you would have a hollow section along both sides of the ridge. Also most likely a visable hump, after installation.