How it works
FargoRate's scale is logarithmic: a 100-point difference means the stronger player is expected to win about two games for every one the weaker player wins. FargoRate's own site states that ratio but not an equation. The per-game chance used here, as given by the Playing Pool newsletter, is 1 / (1 + 2−difference/100), so a 500 against a 400 wins about 67 percent of games.
For the match, the calculator assumes every game is independent and that each player's per-game chance stays the same throughout, then adds up every way Player A can reach their race number before Player B reaches theirs. That is a simplification: it ignores who breaks, momentum and table conditions, so treat the match figure as an estimate built on the rating formula, not a prediction from FargoRate itself.
Using it to set a handicap
Give the weaker player a shorter race and watch the match percentage move toward 50 percent. The FargoRate app has its own charts that suggest handicapped races between two ratings, and leagues that use FargoRate set handicaps their own way, so use those for official play. Read more about how ratings work.