Petrie has 1318 in. (your 1315) from descending passage floor to the south end of blind passage and 1012 in. south of mid line. Also the roof of the blind passage is 1184 in. below base line and a height of about 29.5 in. so about 1213.5 in. for the floor.

So your figures look about right from what I can tell.

RLH