Is it more important for your baseboards to have a consistent reveal (height) or for them to be level?
When putting a narrow item on a wall hang it level. When putting a wide/long item on the wall hang it parallel to the floor or ceiling, whichever is nearer. The eye can see a picture frame askew because it just doesn't &quot;look right.&quot; But something long like a 10 foot long dry erase board, a wide painting.. or trim work like baseboard or crown molding.. will be judged by its parallelism with the adjacent floor or ceiling. Nobody can tell whether the floor or the ceiling is actually level; we all just assume that it is.
Likewise, don't adjust the width of a baseboard so that the lower edge can track the floor and the upper edge remains level. The resulting wedge-shape baseboard will stick out like a sore thumb. Choose some spacer stock (scraps of 1/2&quot; drywall, scraps of the baseboard itself, etc), support the section of baseboard on several spacers to keep consistent distance off the floor, and nail it in place.