cdmurphy

Well, bike fit is kind of a can of worms. A lot depends on how you are riding, i.e. high output racing calls for a more forward position, where you can use your legs to provide upward force to counteract gravity pulling on your torso. That same position when just poking along at 10-12 mph will result in you having to hold your torso up with your arms, and will soon be very uncomfortable.

You might want to try the online fit calculator at Competive Cyclist: https://www.competitivecyclist.com/S...ulatorBike.jsp

It's not perfect, but if you're careful with the measurements, it will get you pretty close. The good news is that if your inseam is 32", and you're 5'10", then you are very average in terms of leg to torso length. That means you won't have to do anything wacky like super long seat posts or really tall stems to get a decent fit on most bikes. I still think a 56 is probably closer to your "ideal" size, but a 54 would certainly work, but might not be as comfortable for long efforts with the lower bars.

Generally, standover height (The height of the top of the top tube) doesn't make a lot of difference to bike fit. It's nice to have enough room to straddle the bike without any uncomfortable contact, but that isn't always possible. (Folks with short legs for their height generally need frames too tall to straddle comfortably). On the other end, folks with long legs like me will have tons of room on most frames, as we often need to ride smaller frames, to get a short enough top tube.

Modern bikes throw a lot of this out the window, with their sloping top tubes. This lets them put the handlebars wherever they want, in relation to the seat tube. But, your Miyata is one of the classic, level top tube bikes, where the seat tube length generally determined where the handlebars wound up. If you want the bars lower, you'll need a shorter frame, and a longer stem to make up for the shorter top tube. Going the other way, if you want the bars up higher for more comfort at lower intensities, then you'll want a taller frame, and maybe a shorter stem if the top tube is longer.

Just where "you" want the bars is the big unknown. Some of this will be a matter of your physiology, and much of it will be personal preference, and your conditioning (core strength, flexibility, injuries, etc). As a rough guideline, the harder you are riding or pushing yourself, the further forward or lower you will want the bars. This will help keep you balanced at higher output levels, and cut down on wind resistance (unimportant at low speeds, but hugely important once you get over ~25 mph)

Also, time in the saddle makes a big difference. Positions that are comfortable after years of riding may be torture for a new rider, or positions that are comfortable for 25 miles, may be agony at 70 or 80 miles. All the bike fittings and formulas in the world won't tell you what is most comfortable to "you". Most experienced cyclists arrive at a good fit through a lot of trial and error. If you go out for a long ride, and something really hurts, then it's time to do some research as to why it's hurting, then try changing something to fix it. Eventually you get fitter, and your bike gets adjusted better, and you reach a compromise that is generally comfortable.

Then it's a great idea to really measure the important contact points, so you can set new bikes up the same, or at least close. (At a minimum, you want to know saddle height - either crank center to saddle top, or pedal to saddle top, saddle setback (distance from saddle nose to bottom bracket - horizontally.), handlebar height, and handlebar distance from either the saddle nose, or the bottom bracket, again, horizontally). With those numbers, it's pretty easy to duplicate a particular position on another bike.