I've read that the average inseam to torso ratio is 45%. (For me, 187 cm height with 91 cm ends up being 48.6%.) I was curious to how much shorter my torso is compared to normal, but trying the math confused me. If I start with the 187 height, my inseam would need to be 84 cm to give me 45%, meaning my torso is 7 cm shorter than normal. But, if I use the 91 cm inseam, my total height needs to be 202 cm, giving me a 15 cm shorter than normal ratio. (At least now I know why 4 bike fitters couldn't come up with a comfortable and efficient position for me!) Any ideas which it is . . . or perhaps I'd have to split the difference? (Forgive me if the answer is obvious, but I'm no mathamatition.)

I think you mean inseam to total height. Inseam to torso of 45% is pretty damned stumpy.

An inseam to height ratio of 48.6 puts you on the lanky side, at least from the hips down. To get the total picture right, you need to look at your arm length, too. In general, long arms indicate a need for a lower, longer reach to the handlebar than torso length alone would indicate. On the other hand, if the arms are more in proportion with the torso (short, that is), a higher, shorter reach is needed.

I'm a long legs, long arms sort of guy, and I wasn't getting any traction listening to the advice of clubmates who were of similar height. Plugging my numbers into the Fit Kit tables was a disaster, and I even temporarily stumped the guy who created those tables. He looked a the bike and scratched his head, until he saw me sit on it, and it was pretty darned right. I'd gotten there by modeling my fit after a well-known pro of similar height and proportions.

There are generally two sizes for riders like us, the total height size and the leg length size. Sometimes getting it right involves splitting the difference and sometimes it means just choosing the smaller one and working it. F'rinstance, a 56 in a Trek is just too small for me, but it works fine for a Scott Addict or CR1, a Specialized Tarmac, or a Cannondale. I'm riding a 58 Trek these days, with a 12 cm stem slammed, and my back angle is still not especially low. Those long arms.

Getting tuned to a position requires some time too. You're likely to feel slower, initially, with the recommendations of a fitter. This is about getting new muscle groups working together.

I don't know where the 45% number comes from, but it may be off a bit.

For example, this write up would indicate nearly 48% as the average: The Difference Between Boys & Girls - Part 1 - SPEED Theory. You may want to look at the chart of inseam vs. height on that page for some idea of normal. For a person 187 cm tall, the write up would, if correct, indicate that a 91 cm inseam is probably within one standard deviation of average, which is pretty close to average (I forget many of my statistics learnings, but if I'm right, 2/3 of people or things being measured are typically within plus or minus one standard deviation of average).