OP,
Check the head on the inset nut that centers your dual pivot calipers. It may be 2.5mm, so you'll need that size, too. I'd recommend buying a good, complete set of metric long L allen wrenches. The Park Tool master chain link removal pliers are terrific, too.

As you attempt more complicated service, you may want to pick up the specific tool needed. That's the way most of us have done it over many years. Just be thankful that you're not servicing old Campagnolo stuff...
PG