On Mon, 10 Aug 2020 14:27:47 +0100, bert wrote:

3 - 5mm ish

It's not a lot is it, from full open to full closed.

Area of 15 mm dia circle about 175 mm^2
Area of 5 mm of 15 mm dia cylinder about 235 mm^2

The two areas are roughly equal with 3.7 mm of cylinder.

--
Cheers
Dave.