The easiest and fastest way to notch the logs is with a jig
My site has plans to build your own if you want : http://logdovetailjig.com
. By building your own, you can have a custom set of jigs that will work for your particular log dimensions and end up with the gap between the logs that you want.
Our example cabin for this instructable is 11'-8" x 16' using logs that are 7" thick x 9" high. The gap will be 2.5 inches. Yours could be other sizes. The plans are custom designed for whatever dimensions are specified.
After you decide on the cabin dimensions, add 4 inches to each and use those numbers to cut the logs to length.
So the logs for this example would be cut 12' and 16'-4".
The jigs shown are for half dovetail notches and take about 2-3 hours to make.