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If you can manage to trace / cut a piece of paper so that it is the exact same size as your primary mirror you can then fold it in quarters and with it still folded cut the barest tiny tip off the "point" of paper that corresponds to the centre of the circle.
Gently and carefully (so as to not scratch the mirror) lay that paper circle flat over your primary and place a tiny mark through the centre hole with a permanent pen.
You can then use that dot as a centre spot although if you are going to be using a laser collimator I'd say try to put a binder ring around it so it's nicely centred. The ring won't affect viewing as that part of the primary doesn't actually affect your view as it lays in the shadow of the secondary mirror.
Cheers!
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