Something fun to calculate with troop drop rates is time to bring a lvl 4 troop to level 30 by farming only.
(0.15 1* troop/run) x 40 exp/1star troop + (0.05 2* troop/run) x 90 exp/2star troop 
(3 world energy/run) x (0.2 chance for specific color) = 0.7 troop exp/energy
It takes ~64,000 energy to max a set of 4star troops of all colors, which would be 91,428 world energy * 10 min/world energy gives ~635 days, assuming you use all your world energy for farming 3-world energy levels.
Note that flasks, level ups, epic troop and regular summon tokens will reduce this. For comparison, I’m just about to finally hit lvl 23 on some mana troops (~38,800 total troop exp). The method above would predict 384 days to hit lvl 23, but I have only been playing 324. So the other sources of troop exp shave some time off, but the bulk of my troop exp comes from farming with world energy.
Another fun calculation is comparing spending gems on troop summons (160gems/pull in bulk) vs world energy refills (100 gems). At level 46 I have 39 world energy, which gives (0.7 troop exp/energy * 39 energy) / 100 gems per refill = 0.273 troop exp/gem
Assuming a troop summon has 5% chance of a 4* you get 0.95 x 200 + 0.05 x 400 = (210 exp*0.2 color chance) / 160gem = 0.2625 troop exp/gem (the 5% 4star rate is a guess. 10% gives 0.275 troop exp/gem).
Based on this simple estimate, at ~37 world energy it costs less gems to refill world energy and farm feeder troops than to level using troop pulls. They are very similar though, and each has it’s advantages. Using world energy refills you get crafting materials and hero feeder materials, but it takes a lot more food (!!) and real life time. And you won’t get any 4* troops this way while pulling you can get a full set of crit and mana troops in each color.
Based on how close these values are, I can’t help but wonder if they back-calculated troop drop rates based on world energy refill costs at a certain lvl vs troop summon costs.
P.S. please check my math and correct me if I’m wrong anywhere.