Istvan, it's easy to understand (U doesn't see JBT last WE ? ;) )
For example, you want tune your board at 90° :
1. tune base edge at 1°
2. tune sidewall with roto-finish 200 at 88° (10 or 15 forth and back)
3. then same thing with roto-finish 400 at 89° (6x max) --> the result angle is 90° with 1° for the base
With this technic, no need to use each time the file, but just 2 diamond at different angle