I want to install a chair rail in our new house. Problem is, the floor and ceiling aren't perfectly level. I started to draw my guideline by measuring down from the ceiling at several points around the room. After doing this I used my level and found the line to be 'not level'. I re-did the line by marking the height at one end and then using the level to draw a straight line. I guess my question is, should I follow the line made by using a level, follow the line made by measuring down, or draw a new line somewhere in the middle?

I'm obviously new to this and would appreciate any advice or opinions. Thanks!