… There's a default example in Rehearsal Marks but I can't figure out how to edit it …

It is not clear to me, in what ways you need to edit the rehearsal mark (?)

Do you need to

1) change the font?

2) change the default positioning?

3) change the starting number

Please explain.

All of the above (1–3) can be done in Finale.

Change the starting number, like I said.

In my own copy of Finale v26:

Expression Tool.

Select the handle on the rehearsal mark number (so that the expression is selected).

Expression menu > Edit Rehearsal Mark Sequence…

Brilliant!
Thanks!