I am putting up chair rail, if I install it at the proper height (32" - 36" for eight foot ceiling) it runs into the apron of one of my windows. I would have to install it at 39" height to go above the apron and sill. I believe that might be too high. Don't know if I should keep it at the right height and end it with a return before it runs into the apron or just move it up above the sill. Removing the apron is not an option. Any ideas or thoughts would be appreciated.
Pete

I would run it above at 39 I've also seen some at 42 and 48, to me going this high is becoming more of a waincott look than just chairrail.
Here is an option, if you not going to paint a separate color on top and bottom then id run the top of the chair rail at 36 and return it a little before the window apron. As far as a return what we do instead of a 45 cut is to cut a 22.5 then the return cut will be a 22.5 and on the other side a 45 degree cut , just gives it a smoother finished look..

Return it down to go underneath the apron.