Actually, I think it's a nice thought to use Jamison if your fiance's name is James. Generally I'd rather see a James, but since Jamison would work meaning wise here.. why not? I think it would be more handsome in the mn spot, as its surnamey. NN Jamie is fine.
I'd love to be able to use James, but can't unfortunately. My fiance has a 5 year old son from a previous relationship and his mn is James, so I don't want to re-use it.