Here is a PDF of the FY17 Medical Corps LCDR promotion board statistics, summarized here:
Above Zone – 3 of 7 eligible officers selected – 43% selection rate
In Zone – 219 of 249 eligible officers selected – 88% selection rate
Below Zone – 24 of 503 eligible officers selected – 5%
OVERALL – 246 officers selected out of 249 possible – 99%
The last line might not make much sense, so let me explain it. First, the according to page 2 of the board convening order the promotion opportunity was 100%. You take the number of officers in zone (249 in this case) and multiply it by the promotion opportunity (100% in this case) to give you the number of officers the board can select for promotion (249). The 99% overall selection rate incorporates the 3 they selected from above zone and the 24 they took from below zone and adds them to the 219 they took from in zone giving you 246 out of 249 possible or 99%.