Sorry, but I've been busy with snow removal and work.
Your author misinterpreted the data. Proficient means just that, it does not include those that are more or less than ranked proficient.
Here's the raw data from the state DPI that collects the data.
Looking at the numbers you want the MinPerf WSAS numbers. Which you'll see are at 9% at the worst for the state level.
Simple math here means that 90% are at least of basic reading proficiency for that grade level, or better.
Thanks for picking an author who can't read and interpret the raw data correctly.
Interestingly enough, if you select only Milwaukee you get a 30% sub-proficient rating. You know, the district that Scott Walker helped run prior to being elected Governor.