I use a chart in the back of my complete guide to bicycle maintenance and repair book, but there's a formula that i have no idea how it works or ever bother to look at closely.

Add something, divide something, that's where i usually fall asleep.

To give u an idea i have a 52:21 which is 66.9 inches on my bike, which is even lower than your original.

But it's all about you, it's your world, your bike.