I still think this is your best method. It calculates the error without guessing on tire sizes, because tire height vs. rolliing diameter can vary by tire design (radial or bias for example).
Find a measured known distance or use a GPS and drive 10 or more miles (more is better). Record how many miles your odmoter registers over that distance. Count your old speedometer teeth. Then use this formula:
New Gear teeth = Odometer miles x Old gear / Known Miles
When I had my 67 Valiant, I used the charts and it still wan't that accurate with the new gear. Then I used the calculation method and ordered a different gear and it was accurate.
So, if you have a 30 tooth gear and you speedometer reads 12 miles over a 10 actual mile course:
New gear = 12 x 30 / 10
So in this example, you'd need a new 36 tooth gear.