Calculate Mind Funeral efficiency
This situation can be approximated by the following model: you have an infinite sequence of lands and non-lands, where the probability P that each is a non-land is equal to the fraction of the deck that is non-lands. So, in a 60 card deck with 36 non-land cards, we say P=3/5.

Now, starting at any point in the sequence, the probability that we see exactly N non-land cards followed by a land is simply P^N * (1-P). From there, to get the expected number of non-lands followed by a land, we calculate

Sum {for all N} of N * P ^ N * (1 - P)


The 1 - P factor is independent of N, so that's the same as

(1 - P) * Sum {for all N} of N * P ^ N


This is then equal to (see Wolfram|Alpha)

(1 - P) * P / (1 - P) ^ 2 = P / (1 - P)


So, the expected number of cards that we mill when we mill until we see one land is 1 + P / (1 - P) (adding 1 to also count the land)

Then we simply multiply that by the number of lands that we mill until.

If we apply this to Mind Funeral targeted at a 60 card deck with 24 lands, or at a 100 card deck with 40 lands, in either case about 3/5 of the deck is non-land, so the expected number of total cards milled is

4 * (1 + (3/5) / (2/5)) = 4 * (1 + 3/2) = 10


We expect to see 10 cards milled from a deck that is about three fifths non-land.



There is a little simplification we can apply to the formula above, based on information we can infer from the initial definitions. Specifically, P is the fraction of the deck that is non-lands, or in other words non-lands/(total cards). And 1-P is the faction of the deck that is lands, so that's lands/(total cards). Then

1 + P / (1 - P) 
= 1 + (non-lands/total) / (lands / total) 
= 1 + non-lands / lands 
= (lands / lands) + (non-lands / lands)
= (lands + non-lands) / lands


And of course the number of lands plus the number of non-lands is just the total size of the deck. So, the overall formula for the expected number of cards that you mill is

(lands to mill) * (library size) / (lands in library)


or, perhaps more clearly

(library size) * (lands to mill) / (lands in library)


In other words, the fraction of the library we expect to mill is approximately equal to the fraction of the remaining lands that will be milled.