Example

3.6% m.f. milk

0.05% m.f. in skim
40% m.f. in cream
0.3% m.f. in buttermilk
81.5% m.f. in butter

Es = 1 - .05/3.6 = 98.6
Ec = 1 - .3/40 = 99.25
% Composition Overrun = (100-81.5)/81.5 = 22.7%
If 100 kg of milk was used, 8.9 kg of cream would be produced (from a Pearson Square mass balance) and 4.35 kg butter would be produced from that. This is the theoretical yield based on no losses. The mass balance of fat shows that 98.3% of the fat ended up in the butter, 0.4% of the fat ended up in the buttermilk and 1.3% of the fat ended up in the skim.
The % Churn Overrun = (4.35 - 3.6)/3.6 = 20.8%