You have to understand that black holes are living things--and like all living things, they like to eat. And when they grow, they tend to grow exponentially. As the thing grows, its power grows as well; therefore, we shouldn't expect the suck-in rate to be flat. It's diameter will increase, and its gravity will as well. Eventually, it will cause cavitation; then the "solid" Earth will immediately collapse on the thing, force-feeding it. So I took your growth rate of 0.4% per half hour, and just extrapolated that.