Iride01 

I'm 180 cm or a tad less with 87.6 cm inseam. I use to ride way oversize for me frames with comfort. Not just 12 years ago I had been on a 25" ( 63.5 cm) bike for over 30 years. However in my older age I'm finding smaller frames better and have progressed down with 59 and 60 cm bikes to a 56 cm bike that is currently the lower recommendation out of the two sizes I was between. And I think I could have gone to a 54 cm bike, but probably that would have needed to be a different model with higher stack.

So while yes the manufacturers do build for the average proportions of what they feel their customer is, you should be able to find something that fits you well if you realize reach and stack as well as other geometry are different for different models of bike.

If you go with a smaller bike of the same model, then your stack height will get lower as the bike size decreases. And since on a road bike geometry your saddle height will always be pretty much the same distance from the BB or pedal, then your bar drop from the saddle will be more. Reach also decreases, but I've not found reach as big an issue and that can more easily be corrected with other adjustments.

So for the Tarmac SL6 in a 56 cm frame right now, if I was to go to a 54 cm frame I would probably be too aggressive a position for me with a lot of bar drop from the saddle. I might could just keep a lot of spacers under the stem, but a Roubaix with a more relaxed geometry and higher stack would probably be a better choice in a smaller size than recommended frame.

So to me, trying to figure out the correct size of bike to get form number calculations is silly... unless you already know from experience what works for you. You really need to just try on different models in different sizes and probably even a size out from the manufacturer recommended sizes. And remembering that a particular size, say a 56 cm bike in one model might be entirely different fit than a 56 cm of another model. Again, primarily due to reach and stack differences. Some go by horizontal top tube length instead of reach, but a road bike is going to have pretty much the same seat tube angle within a few degrees to make that a insignificant difference for whether using TT length over reach.

As for long legged people needing higher bars and shorter stems, I can't agree. I went lower on the bars when I got my 56 cm bike than I ever was on my other bikes. It's really just a matter of getting use to a new position. And that does take some effort. It's a tad uncomfortable at first to have your head so low and feel like it's the battering ram for whatever you might crash into. But you do get over that. At least I have.

I did right after I got my bike swap the 100 mm stem for a 70 mm stem of the same angle. And I thought it helped. However I just put the other 100 mm stem on about 4 or 5 rides ago after using the shorter for a long time, and I'm not really sure that I see the difference I thought it made. The 100 mm stems seems to be doing just as well for me.

Another thing I'm finding as I age and have entered retirement is that my muscle strength and mass is decreasing in my arms, hands and elsewhere on my body since they don't get used as much. And I'd have the odd hand numbness where I never had it before and other odd pains. As I've always found even when younger, that exercising those muscles and keeping them toned up... not necessarily building more muscle in those places that bother me really seems to help with aches, pains and numbness of things when on a bike. I keep a set of those hand exercisers with the springs next to my easy chair to help keep my grip strong so I don't get numb hands. Though the position of your STI's and angle of your drops as well as width of your handlebars makes some contribution too.

As for large frame bikes stretching out your hand position far in front of you, that does work well if you like the lower head first battering ram position. I did and being stretched out prevented shoulder, elbow and hand pain. With the smaller size bikes I'm riding now, I'm finding I have to keep a good bend in my elbows to avoid the issues with shoulders and elbows and hands. And for someone that spent most of their life with straight outstretched arms, that's taken some work on my part.