Originally Posted by grebletie
I'm not sure what back strain would indicate, but have you tried your inseam in cm X .883? That gets me really close to where I need to be.
That would be measured from the center of the bottom bracket axle to the top of the saddle following the seat tube. The measurement works in inches, too, not just centimeters. (See below about accurately and properly measuring your inseam.)
Other ways to find the correct height for you are: measure your inseam by standing in stocking feet with feet about eight inches apart and pull something like a 1 x 4 board held horizontally up against your crotch as tightly as possible. Measure from the top edge of the board to the floor for your inseam measurement. (It is not the same as what is listed as the inseam of your trousers.) Then multiply your inseam figure by 109% and measure from the top surface of your downmost pedal (with the cranks in line with your seat tube) to the top of your saddle following the line of the seat tube.
Some like to get on the bike and raise the saddle until their heel passes over the pedal at the bottom of its stroke with about a quarter inch of space to spare. This would be with your leg fully extended. This provides "room" for you to ankle when riding and still not have your leg extended straight out, but retain a little bend in the leg, as recommended.
Regardless which of these methods you choose, try altering the seat height about a quarter inch and riding on it for a few days to find what you most like. I find raising the seat gives me more power, but too much height makes it harder to apply power from the 11 o'clock pedal position on over the top, and a seat position too high tends to cause chafing in my groin area. The fabric on my pants lies differently when the seat is a bit high and introduces stresses even without my hips rocking. So, the correct seat post height is a compromise between power and preventing chafing, at least for me.
I also record my preferred seat height so I can duplicate it later. And, my carbon seat post creeps downward even when the collar is properly torqued. So, I moved the mounting collar for the seat post reflector down to the seat post collar and it not only stops the creeping, but provides a marker for the height of my seat post.