What is the best way to measure for a chair rail going up a flight of stairs?

Really hard to do (chair rail going up the stairs) and not a really good decorating idea. But the same rule applies, four feet off the floor, but you will never get it looking right because it is going up the stairs.

If you measure from the front of step 1(point A) then the front of the last step(point B) the same height,strike a chalk line at point A to point B it will be the same height.