i normally take into account which tiles and how many rows are goin in when setting a height for wall units. the "standard" height is about 18 inches,ie 3 rows of 6" tiles. sometimes tho if the customer has chosen 8" tiles ill set them at 16" ie 2 rows of 8" tiles. or add another 2" if theres a small row of border tiles,either way,for practicallities sake i certainly wouldnt set them higher than 20" at an absolute max!
If you have a full length unit or a dresser unit, use the top of this as your overall height and keep wall units same height! usually kitchen units are 2120mm high, so the top of your wall unit should be 2120. SO
2120 - 720MM WALL UNIT = 1400mm to bottom of wall unit
most worktops are 910mm high, so gap would be 490mm