Soak the mix only as often as is needed to keep the potting mix (bark) moist. I wouldn't let it dry too terribly much in-between waterings, since the plant is in recovery.
The yellowing leaf could be due to a number of things. If it were an issue with the amount of light, though, you'd see burn marks on the leaves (starting as yellow spots, then turning black and dry), rather than a gradual yellowing at the base of the leaf.
My guess is that the plant is showing a stress response due to the condition that it's in. If the leaf falls off, don't worry about it, and just keep on with the watering schedule.
-Tim
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