The standard deviation is 0.8 for a single HLL, but a ratio involving 2 of them will have about 1.6%. Simple arithmetic: conversions x (1+0.008) / [visitor

The standard deviation is 0.8 for a single HLL, but a ratio involving 2 of them will have about 1.6%. Simple arithmetic: conversions x (1+0.008) / [visitors x (1 - 0.008) ~ (conversions / visitors) (1 + 2 x 0.008 + ...) ~ (conversions / visitors) (1 + 0.016 + ...) (It should actually be a bit less if you do the calculation carefully, probably 1.6/sqrt(2).) 12kb is also a pretty big filter, but I guess reasonable. It's definitely true that non-statistical errors are highly likely to drown this out. But why have extra error if you don't need it?