Here's some tips I posted on another forum:
The lock miter joint is great for exterior work but I used to resist using it because of the setup until I started doing the following.
#1. Start off with material that is of uniform thickness and jointed before starting. Even for exterior work, I make sure that everything is of uniform thickness. I recently did a job with Azek that varied in thickness and it was pretty frustrating and starting with material you've jointed and planed helps.
#2. Mark the center of a piece of test stock and set the bit height to that mark. To find center, I use a marking gauge using the mark, flip piece over and mark method. Compare the two marks and if there's a pair of parallel lines, reset the marking gauge and do it again until you leave a common center line. As to bit height, the trick here is that what many think is the center of the bit is not actually the center. That's because the tenon produced by the bit has a sloping face on the top side. Center of the bit is actually the very corner of the bit.
#3. Before making any test cuts, set the fence slightly forward. To set the fence, position your test stock vertically against the fence and slide it up to the cutting edge of the router bit. When you sight across the router table surface and compare the bottom cutting edge of the bit to your stock you will see roughly where you have to position your fence to produce a crisp mitered corner. Don't go for perfect on the first cut. Move it slightly forward and lock the fence down.
At this point, you have the proper bit height and the fence roughly where it needs to be set.
#4 Make a test cut in the vertical position. Be sure to use a backer/support board if you're cutting stock that is narrow. Inspect the end of the board's mitered edge. If the mitered edge looks blunt/square, move the fence back slightly and make another cut until you can produce a crisp, sharp mitered edge.
If you've done it right, you should be able to run a piece flat and a piece vertical across the bit and produce the joint with a flush mitered corner.