I may be slightly wrong but here what I think it is...
1, strike datum line around room (must be where a horizontal joint will be).
2, mark out full tiles down to the floor until you you can't fit anymore.
3, measure from the datum line to the bottom of the last tile and subtract joint width + a few mm... Let's say 5mm in total, now cut a piece of wood to this length
Spot on Mark.
5, take the piece of wood and place vertically on wall with one end touching floor, place a mark on the wall at the top of the wood. ... Continue this process at where every vertical grout joint will be (very important you insure the wood is vertical during this stage so use a level along with wood.
6, draw a vertical line where each tile will be connecting the 2 horizontal lines
you should now have your bottom row measurements.