I just finished my third lower and learned a few things along the way.  All my lowers were bought from 80%lower.  

1.  My lowers came with the rear pocket partially milled.  I believe you can leave it as is or complete it like I did.  At this location is the support screw.  It is normally not installed until the very last.  I left mine in for the most of the operation to aid in alignment.  You do need to remove it to mill the rear shelf.

2.  The manual says to make sure your mill end is very tight.  This is true, as a matter of fact you need to tighten it as much as you possibly can.  I mean really effen tight.  If you don't you'r mill end will creep out and cut too deep.  You could mess up a nice lower if you don't watch this.

3.  The directions say to mill about 2mm a pass.  I did this for my first two lowers and they turned out fine.  For my third I milled 1mm each pass.  This takes twice as long of course but you'll end up with a piece of art.  It turned out outstanding.  I'll do 1mm from now on.

4  Through out the operation keep checking for loose fasteners.  The drilling/milling can loosen them up.

5.  Do not allow the drill stop to contact the drill template.  If you do you'll risk the possibility of the drill stop sliding up and you'll drill to deep.  

6.  I'm using the recommended Mikita.  It has 6 speed adjustments and I found 3.5 to be about right.  I have no idea what rpm this is.

I hope this helps.

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Thanks for the pics.  I've got the Modulus jig and a couple of lowers to build. Now, I just have to get the workbench cleaned off. That shouldn't take more than a year or so.

I've used the Modulus Jig and American Gunsmithing and Tooling end mills https://americangunsmithtooling.com/products/1-4-0-75-short-flute-solid-carbide-and-zrn-coated-end-mill-100-made-in-america. Those end mills cut like butter. The Modulus Jig holds up through dozens of lowers if you take your time and use a drill press for roughing out. Don't bother trying to mill the shelf if it's already cut out. Your endmill will bounce around and you could risk it coming loose.

Good info on the mill ends.  I bought 4 of the Modulus ones so I'm good for now.  Even though we went slow on the first one the cutting edge chipped off on 2 of the flutes.  I can't imagine drilling a lower without a drill press.