But according to the Retul, this is the stack and reach without spacers.

So if I understand, you are saying that if the bike is too short, add spacers to jack up the effective height and add a longer stem if the reach is too short.

OK. So as long as I do not go over, I could accommodate, theoretically.

Let's look at a Cervelo R5.
I am a size 58 on my current bike. The 58 R5 has a stack of 605 and a reach of 396. Now these are not to the handlebars but to the top of the head tube.
A 56cm R5 would be stack: 580 and a reach : 387

My Frame stack and reach are 625 and 383. If it matters, a Specialized Roubaix.

Since the reach on the 58cm R5 is 396, either I would need to be much more flexible or grow. Not sure it would work for me but it would only need 2cm of spacers to get the stack. I would need one hell of a lot of spacers to get the 56 R5 to work (4.5cm) and a little shorter stem.

What I find interesting is that at 6' 1", I was really sized at a size 59 based on the computer measurements. How is it possible the 56 could be the right fit for me? I am not doubting you but only questioning this because when I got onto a 58cm R5, it was a nice fit.

for that matter, it doesn't look like any bike in Cervelo will fit since they appear rather similar in stack and reach ranges.

Sorry, I am new and rather ignorant to much in the bike world and really appreciate the understanding.

Thanks