It's not that much of a grey area. The ATF has published regulations that require the fire control control pocket be created before a lower receiver is legally considered a firearm. The people who have gotten in trouble here have created lowers where the fire control pocket has been started in some way. In one example, a company created a "biscuit" of one color plastic in the shape that needed to be milled out and then shot the rest of the lower around that biscuit. The ATF argued that the fire control pocket was created in this process and the end user was just removing a plug that was inserted into that pocket. In other cases, the manufacturers made index markings showing where to mill things out.

If you stick to an 80% where the fire control pocket has not been started and you select a lower that requires a separate jig to mill, you'll be fine.