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.