Any of these help? :
John Clements Sportster is fitted with a J type trim: it is a little more forgiving of non perfect panel fit.
Although the last one is my Cabrio, I chose to fit my bonnet flush with the nose cone surface, by fitting a thin flat strip of neoprene o0n the landing edge of the nose cone, rather than a J profile rubber strip. It requires more attention to detail, but looks neater when finished. The centre hinge will require a rounded end added to it to cover the "sharp leading edge" for IVA.
Do not try to measure the centre - you are best using your eyesight and judgement, as nothing on a Marlin is symmetrical.
I found the best way to resolve the bonnet pieces was to set the gaps between the top of the side panels and the bonnet with spacers of your chosen depth, then spend time gently forming the bonnet shape to fit the nose cone, such that without any weight applied it follows the nose cone perfectly. Then trim the front edge to follow a parallel line with the nose cone. Finally you have to be brave and trim the hinge edge of the bonnet panels to the hinge. As Robin says, leave 2-3mm gap between edge and hinge, to allow paint to fill some of this, and make sure it does not rub when finished. This is still more difficult than it sounds as there is nothing to support the bonnet until it is attached to the hinge - Catch 22!
I used stainless screws that had plain heads, and despite what one might expect, they tighten up easily.
Simon Gregory used counter sunk pop rivets in order to make them flush with the bonnet material, which worked nicely, and were covered over by primer/filler paint: the only draw back being that after a while the filler paint cracks. Moral being, don't try to hide your bonnet fixings!