Q.Want To Transplant A Little Hemlock. In NW MI, Lower Peninsula, A Little North Of Middle Of West Lakeshore Area.
Is there a big diff in early Sept vs spring timing? How do I ‘measure’ how wide the root spread that I must dig in order to move the tree successfully? I have read different ways to estimate that.

Certified GKH Gardening Expert
This will be best measured by the tree's caliper. Here is an article that will help:
https://www.gardeningknowhow.com/ornamental/trees/tgen/transplanting-trees.htm
It won't really matter when you transplant, just that it is mild when you do. There are some pretty specific things to know about the location that it needs to be settled in, though. Here is an article that will help:
https://www.gardeningknowhow.com/ornamental/trees/hemlock/growing-hemlock-trees.htm