Approximately 90 cm from the floor is a pretty good height. It depends on how the handrail looks. But try to hold the rail where you want it with the brackets on, mark on the first bracket, up or down, and measure 90 cm from the floor/stairs. Secure the first bracket, then you have a fixed point to start from when you fasten/mark the other brackets.
(Gosh, there were a lot of F's in that Explanation)
Good luck!!