Hi,
The calculator Tom mentioned is pretty cool. You'll need to know the reduction provided by your OD, to figure out "5th gear", though. A-type ODs mostly provided 22% reduction. J-type ODs, 25% reduction. Some were as much as 28%. Most were labelled on top, the first two digits of the model number indicated amount of reduction, i.e.: 22xxxxx or 25xxxxx. Of course, there is a possibility that any given OD could have been changed to a different reduction ratio at some point during it's life. But, if you know its history, maybe that's not a concern.
Cheers!
Alan