Yes it's still rather impure but 69rons d-limo TEK is known for that. I'd honestly forget about the MEK and just dose it as is..
And no one can really tell you how much to take because cacti varies sooo much. Unless you have rather pure Mescaline (which can be obtained fairly easily with other TEKS) where you can see some crystals, it's a crapshoot on exact dosage. It's the only drug I go into with zero expectations on strength of experience.
And I've been growing my own Pedro for the past 9yrs or so.. Cuttings from the same plant can vary based on time of year, how much it's been watered, how much it's been cut on, the list goes on. (Tip: I find stressing them does increase alkaloid content. I don't do this intentionally but plants that have been cut many times seem to be stronger. Also I believe the plant concentrates the alkaloids in the new growth so I just keep cutting off the new growth usually around 6in. With my small garden I can trip quite a few times a year. Most of mine are PC's I believe but with this growth technique I've gotten much stronger cuttings than originally felt when first obtained. After trying bridgesii, I prefer my Pedro's I think..)
-GC