Note that 5.04 / 0.36 = 14. So the natural healing of the light wound + the [6] HC was 14 times that of the [1] HC; this means that the natural healing rate of a light wound appears to be 8x that of a [1] HC.
Continuing tests, light bruises and bites typically healed at this same rate. While there were outliers, they might be from hits that I didn't see.
I then tested with priest-alt Fobro. I got a medium wound, confirmed that it was stable by waiting 10 minutes. I then applied a [1] potency HC. The next three healing ticks the wound dropped .49 each tick, until the wound was at 14.76 (light). The wound then healed at 4.41, until it got to very light, then healed in the next tick. 4.41 is 9x 0.49, so the light wounds on Fobro would also naturally heal at 8x the rate of a [1] HC.