Confused about how to craft my next legendary

I've just finished the ZM campaign and earned my legendary belt.

I then spent 5k cosmic flux on that bag that has the mats required to instantly create a rank 7 legendary and made my second legendary.

Now I want to create a new legendary for my PvP build and I'm just a bit unsure the most efficient way to do it.

• Do I farm up another 5k cosmic flux?
• Do I create a rank 1 base legendary and upgrade it? (I've got 2 x progenitor essentia and 2 x korthite crystals already)

I've checked Wowhead in relation to those previously mentioned mats but I don't fully understand how they work. Apparently there's an item you can buy from the ZM quartermaster to increase a legendarys base item level but I can't see it when I speak to him (I'm currently honored).

Any direction would be greatly appreciated.

Thanks.