Setting tire size with DRB II
 

I have a modified 1994 Voyager that I use as a transporter. The Car Camel | Michael's Austin Healey Blog
In order to increase the ground clearance I have fitted 205/75 R15 front tires but that has produced a speedo error of about 5%.
I have a DRB II and seem to remember that this tool can be used to adjust the speedo pinion ratio but I cannot remember how to make the change.
Can anyone advise?

Thanks.