
A good question, Jeff, and welcome aboard KivaFriends!
I think the question of how much to donate to Kiva is a very personal matter. Yes, Kiva needs operating funds, and yes, you have limits as to how much you can donate. I suppose that since you're putting in a regular amount of capital each month one option would be to donate a percentage of the capital you're putting in, and then relend without adding a donation. So, for the example you give, you might consider adding your new $100 each month, plus some percentage (10%? 15%?) of that, $10 or $15. Then when the repayments come in, you could relend, knowing that you have already donated a percentage of that capital. Or choose a fixed amount ($25? $50?) to donate each month to Kiva regardless of your lending activity.
The question is ultimately one of balance, within your personal budget. Whatever you decide to contribute, do so cheerfully and not out of a sense of guilt or obligation. I think if you can manage that, then the amount won't be as important, and I suspect the balance will be right.