I use RemoteMaster, and let it do the calculation for me. An EFC is an encoded version of the data that is sent to the executor (a subroutine that makes the pattern of light flashes). For simple IR protocols, the OBC is usually very similar to the data fed to the executor--perhaps the bit order is reversed and/or the zeros and ones are flipped. In some cases, the IR signal includes checksums or toggle bits, and some executors are able to send multiple styles of signals, which then requires control bits in the input data to the executor. These all require more data than just the OBC. Keeping track of all this is a trial, but RM makes it relatively easy.

The primary use for upgrades is to upload the upgrade via a JP1.x cable. However, RM will show you the EFCs for an upgrade and you can manually enter those (but with a little less experimentation!)