Best bet is to just use the heights of the decks above waterline, and add & subtract the rest. The numbers can be found here:
TIP | British Wreck Commissioner's Inquiry | Report | Description of the Ship
For instance: the figures say that E deck was 11ft above the water line (34ft 7in) amidships. As can bee seen in the plans above that's the first deck above the reciprocating engines, so 45ft 7in (34ft+11) above keel. Then take away the height of the tanktop which was 6ft 3in (75in) in this area, (63in in the rest)... so 45ft 7in - 6ft 3in = 39ft 4in in height.
It's the long way, but rewarding lol. Perhaps someone actually has the figures. I remember a chart in one of Sam Halpern's papers or book (Centennial reappraisal) that had a graph showing deck height vs. water or something. Perhaps take a peek at his website too...
Titanicology