Revo BL Gearing Question

Hi all - This forum is the main reason that I decided to convert my Revo 3.3. After buying the following:
Feigao 8XL
MM 1/10 esc
6000mAh, 7.4V, 2 cell

My question is what pinion should I use if I plan on keeping the stock spur gear (38T)? Specifically, I am looking at what pitch I should get as I think I'll just go with 18T.

I went to buy a pinion and there are only 8 billion different pitches. Of course, I was unable to find the answer to the question I'm posting for the forum.

Thanks.