they are an indication of whether the environment is, at the very least, optimal for implantation. This is especially true at 6DPO since that is the beginning of the implantation window.
Also, further down the DPO line, a single solitary value can't be interpreted as being PG. It can be assumed that it's a good sign for PG, but it can be used as a replacement "test" for PG. The only way that you could make good assumptions based on P4/E2 values is by doing serial testing (that's what I used to do). You have to see the trend in those values across several days in order to be able to make good assumptions. That's what I would do (because I was crazy); test every other day. I would also test FSH and LH.