WWI British MkI Brodie Helmet Restoration

[Flying10uk]

I was taught that as a general rule of thumb, the amount of free rivet that you allow sticking out of the metal equates to about the diameter of the rivet. But in the real world sometimes a little more or a little less is required depending on what style of rivet head that you intend to form.

Later edit. How can you tell the difference between a rimmed MK1 or MK1* helmet shell of WW1 vintage and a MK2 shell of WW2 vintage, ignoring any liner which may or may not be present?