Details about score calculation model

[xelxebar]

This is just pure curiosity, but I'm curious about the actual score calculation model used.

Digging through the code, I see it boils down to this calculation, from Score.kt:44:

val multiplier = 0.5.pow(sqrt(frequency) / 13.0)
var score = previousScore * multiplier
score += checkmarkValue * (1 - multiplier)

For an everyday habit, frequency == 1, and with a perfect record, I see this calculation boils down to score = 1 - multiplier.pow(days). And if you suddenly quit from a score of 1, then it boils down to multiplier.pow(days). So the extreme cases make sense. In particular, however, is the 13 constant chosen just to give the particular "3 months to 99%" curve? Or is there more to it?

Digging back through the history, d3b540/Score.java:71 shows a slightly different multiplier calculation.

multiplier = Math.pow(0.5, 1.0 / (14.0 / frequency - 1))

The 1/(14/frequency - 1) curve is very different than that of sqrt(frequency)/13. Any insight on what motivated this change? The relevant commit message doesn't shed much light on it, AFAICT.

Thanks in advance!