Bike fit is very subjective and what works for you may actually be different than someone else with exactly the same bone/joint measurements, and inseam is only the most rudimentary of these. If you really want to find your "sweet spot", you will first need to get your seating position "in the ballpark" (with the help of a professional if possible), and then you will need to put in some miles over different terrain at different paces (while taking detailed notes if possible). It also helps to have a friend pace along side of you while you are riding to video tape you from the side and from the back while you are spinning at around 100rpm or so and grinding at 60rpm or so. Video taping really helps because even your own subjective evaluation of whether your hips are rocking or whether you are reaching full extention will be skewed by your own motion.

Do your hips "rock" when you pedal or do you feel that you are extending you foot or toes when pulling back across the bottom of the stroke? Where on the seat is your weight centered front-to-back and what is the angle of your seat? What size shoes do you wear and where are your cleats positioned on the shoe in relation to your ankle's pivot point? Is your bike set up with a lower stem and handlebar position that has you leaned forward with your pelvis tilted forward, or do you have a taller handlebar position that has you seated more upright with very little pelvis tilt? What is the "Q" factor and size of your crankset? What is the "stack height" of your pedal-to-shoe combo (the distance from the pedals axle center to the base of your foot inside the shoe)? All of these things can effect your exact seat height "sweet spot" so no "rule of thumb" will do you any good except to get you in the ballpark.

Go to a reputable shop with someone who specializes in bike fitting. What they will be able to do is get you "in the ball park" (although another fitter might set you up a few mm higher or lower or more forward or rearward), because it is important to have a good starting point. Once they get you in the ballpark, try adjusting up and down and front-to-back in 1-2mm increments and note any hip rocking, any side-to-side knee movement, whether you feel you are extending your toes at the bottom of the stroke or whether you can "drag your heel" through the bottom as though scraping mud off your shoe. Take notes. Do you feel any slight pain in your hip joints, your knees, your the bottom fo your quads, or your calves? Are you comfortable with you leg extension and hip stability?

Once you have found your "sweet spot" on the bike, document everything. When you move to another road bike or a mountain bike these measurements will get you in the ballpark again, but you will still need to work to find your exact "sweet spot" on that bike as well. How "perfect" your seat height "sweet spot" is depends on how much real work you want to put into finding it, but even a more casual approach will get you to where you are able to apply the most power with the least potential for injury.