Hey guys, just picked up a kit over the weekend. I'm heading to a LHS to grab a few parts and I was going to get a few pinions. I am going to be running a 2100kv motor, any ideas on where I should start with pinion size? (Would be helpful to know for the 43t stock spur and the 45t metal spur as I plan to upgrade to the metal spur)

Any advice is appreciated!!