You want that real close to level and smooth.Having to do some flooring repair, and the subfloor and underlayment seems to have changed (slightly) over the decades. The best I can do is get something close enough, and I'm doing some minor levelling using an extra layer of tar paper as needed. The question that's been raised is "how close is close enough?"
We can feel a slight difference in level between the old underlayment and new. How far out can things be before causing a problem for linoleum? finer than 1/32"?