The advice above is good and is usually the best starting point for setting saddle height, but depending on things like the size of your feet, your pedaling style (toes pointed down during the stroke, flat footed, etc.), your cleat adjustment (assuming you're riding with cleated shoes and pedals, otherwise your foot placement relative to the balls of your feet over the pedal spindle), your hamstring flexibility, etc. your position could vary a bit up or down from that starting point.
A couple of good rules of thumb to pay attention to over time:
- If your hips rock up and down with the pedal stroke because you're stretching to reach the pedals comfortably you're saddle is too high. This can be an issue if you have tight hamstrings and calf muscles even if the height matches the heel test or some of the accepted formulas out there.
- If you develop knee pain in the front of the knee, as in under or around the knee cap your saddle is likely too low.
- If you develop soreness and pain behind the knee joint then the saddle is likely too high.
FWIW, I'm 5'10" tall with a 32" inseam and ride with my saddle ~91cm from the top of my pedal spindle measured in a line right up the seat tube with the pedal at the bottom of the stroke (not at 6 oclock, but lined up with the angle of the seat tube).
BTW, I agree with the post above. You may or may not need a seat height adjustment, but back issues are usually related to handlebar height and reach as well as seat setback rather than seat height alone. Check out some of the information on this site: http://bikedynamics.co.uk/guidelines.htm
-Dave