What is the best position for bath rails?

700 mm above floor level is a good height for a horizontal grab rail.
A vertical grab rail (typically 600 mm long) would start at 700 mm and finish at 1300 mm above floor level.
Don't know where you are located but you could check your Building Regulations (UK) or if you're in Ireland, Part M of the Building Regulations- Technical Guidance Documents - this gives recommendations for facilities for the disabled - which you could apply.

J
February 2008