The arena is 60 meters long & 20 meters wide.

The end letters (F, K, M, H) are 6 meters away from the end of the arena. All of the other letters are 12 meters apart:

Here are some mnemonics to remember letters:

Main letters:
AFBCEK: All Fat Bay Mares Can't Hardly Ever Kick

Secondary letters:
RSVP: use RVSP

Interior letters:
DLXIG: Dirty Little X-rated Inspector General

Diagram of some of the 20-meter circles: