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I was 5'10", but I've probably shrunk a bit - this is caused by compression of the spinal discs, but I guess my legs are the same as ever.
Yes, I measure frames from the centre of the BB to the top of the seat tube, but in the case of a lugless frame with a projection above the seat tube for the clamp, I would measure to the level of the top of the top tube. It's worth remembering that virtually all old British built frames will have been built to a round number of inches or half inches - it would be rare to find one that was say, 22 and three quarter inches, for example.
Bike fitting does depend on the opinion of the person doing the fitting. I had one done before deciding on the last frame I had built, but the result was a bit disappointing. The 'expert' had decided that I was a fast pedaller and recommended a 75 degree seat tube - I never did a decent ride on that frame. Of course it might have been due to my own failing powers.
In the end you just have to find what is most comfortable (of efficient ) for you at whatever purpose you're using the bike. Alternatively if you want to look in period you may well have to put up with some discomfort and inefficiency.
clubman-
Re frame sizes and bike fitting ... thanks @clubman. Agree with all that. The bike fit I had done early on when I picked up cycling again. I used that as a template for other builds but I've since ridden a lot and think that's why I got to wanting a longer reach / flatter position. However I have not ridden much in the last year or so (spending several months of lockdown without a bike or just riding fixed around a shorter circuit), I've even had to resort to running grimace
[The builds I've got going on now are varying sizes with stems of different reaches available so I'm going to play around and see what's comfortable.]
veloham
Thank you @clubman, do you mind me asking how tall you are? I'm guessing you're talking ctt as your measurements are in inches.
I'm 5'8" and my modern bike is a 54cm ctc and that seems about right. I did have a bike fit a few years ago and if I remember, they thought it was ok, certainly can't remember being told it was too big/small. [The main take away point I can remember is that the 172.5mm cranks the bike came with were at the top end of what they'd recommend and I now have 170mm on pretty much all my bikes.]
That bike is square (horizontal top tube naturally) but feel I could have a longer top tube / stem projection than the averageish I've got (I think from memory 9cm give or take a cm). Also think I have about two fists of seatpost showing, although we are not together at the moment sob. Long story short, I think it gives some scope to have a slightly bigger frame with less seatpost.
The ctt measurements give a bit of variability, for example the lugless Hobbs frames have nearly an inch of seattube above the toptube for the clamp. I think the one I have is 23" ctt but the ctc I measured was 56cm.