I really don't know what gives you the idea that you are "between sizes." I am 6'4" and with a 34" inseam. My bikes are:
1: A 60 cm Trek Emonda,
2. A 60 cm Trek Madone
3. A 58 cm Colnago CLX3.0
4. A 58 cm Lemond Zurich
5. A 64 cm Backroads Touring bike.

Now if you are a professional racer, it is pretty important that you have exactly the correct size that would center you perfectly over the center of mass of the bike. The precise measurements is very important because these guys literally live on their bikes so changing the position on the bike can cause them muscle damage. So this is so critical that they now use digital readout measuring tools for everything from the seat height to the damn tilt of the saddle.

But this doesn't apply to people who put in less than 5 or 10,000 miles per year. Saddle height is important only insofar that if you are off more than a couple of percent you get knee pain. If the reach is incorrect you get shoulder, arm and hand pains. None of this is serious and you just play around with it until it is correct. Even crank arm length from 170-175 mm is not important in any way other than saddle height since this is measure from crank BDC to to saddle surface.

If you intend on being a pro racer by all means get a custom measurement and follow them to the letter. Otherwise there are very few measurements that make much difference in anything other than your comfort. Saddle position is adjustable, stems are changeable. Handlebar width is changeable. Old steel frames I used to ride a 62 cm