Your method of measuring does sound about right to me- but don't worry if your bike comes up the wrong size. Frame size doesn't bother me- or rather the quoted frame size. What does worry me is how it fits.

I have found that I like a top tube length of 535mm. With that length TT- I can get the saddle position correct on Fore and aft and can get the reach right with possibly a change of stem length--So to me that is my critical length for a bike. On Boreas that equates to a 51cm frame and on the Giant is an "S" frame. But on other manufacturers I have found it can be as low as a 49 or as high as a 55.

Then you have to take in the peculiarities of how a rider likes his bike set up. I know of two riders- One my size- height and inseam that reckons my bike is too short. And another who is 5" taller than me that other than raising the saddle by 2"- can ride my bike and he says it fits him perfectly.

So does your bike fit you and is it comfortable? If yes to both these questions- then you have the right size bike for you. But may not be for other similar sized riders.