I agree with you, its trouble some. I ask for a tape measure tool that you could layout guidelines that you could snap to while drawing, but I got no traction.Feature request - tape measure with guides
What I do that helps me is to draw a line or rectangle that I can use as a guild that I can snap to while I draw. I hope that helps. Also I count grid square as way to dimension while drawing. On my Mac the grid size is in the upper right hand corner.