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The automatic refill rate is a function of the market cap (mostly price) of STEEM. So it really depends on the price of STEEM going up to increase it, or a fork that increased the share of inflation going to SPS. At the same time, a decrease in the price of STEEM would reduce the fill rate.

I didn't include beneficiaries or donations in that rate, only the automatic refill. I believe these to generally be small (apart from a one-time donation from Steemit Inc that helped jump start the fund), but that could conceivably change. For example, there is currently a high trending post (advert) with a bunch of paid votes and 100% SPS beneficiary. If such a thing really caught on, then I guess there might be more funding consistently going into SPS.

Working the budget to be sustainable will be all important. We will have to work with what we have.

Hopefully, of course, one outcome of the marketing would be some upward movement on the Steem price.