Just had a couple of free days to do some testing in the hope of breaching the Leadership Puzzle and progress has been made. Not enough to make this a full-blown guide just yet, but enough to throw together a rudimentary spreadsheet to play with.

Optimum Army size

As many have guessed, Leadership (Ld) allows heroes to control a certain amount of troops without penalty, here's how the optimum number of troops (Opt) is calculated:

Ld multiplied by 10, plus 100 then multiply the result by Ld again

i.e. (Ld*10 + 100)Ld

e.g. for a leadership of 70
(70*10 + 100)70
800 x 70
= 56000


So with 70 Ld a hero can lead 56,000 troops. Maximum number is per troop type/stack - not total army size, so he can lead a combined force of 56,000 archers and 56,000 warriors without penalty, for example.

Once the number of troops goes above the Opt the hero starts to suffer a penalty to his stats (only the effect upon Intelligence fully tested at this stage, apply to Attack at your own risk). The penalty is approximately -1% per 1000 troops above the Opt. I say approximately because the deduction is in increments rather than precisely applied per single troop above the Opt (e.g. a single troop above the Opt can give a -1.5% penalty but it evens out in the end), the calculator is currently set to be precise - it's too accurate to be accurate as it were

Limits to date
I've focused upon the penalty to Intelligence as it's much easier to do Int tests in valleys, for Attack tests it helps to have a friend to attack. I'm limited to a 125k max troops, my one HH is over 130 Ld so no help from her. Pvp tests are so far inconclusive, I've a sneaking that there's a fixed penalty when fighting other players but there's a lot of work still to do in that area.

When testing your heroes' Ld against valleys, kill the enemy troop stacks in a single round so the results aren't effected by unknown factors i.e. valley heroes' Intelligence - note that they don't have any Attack so input as zero and leave their Ld as 100.

Test Calculator here

Stick to using the Green boxes for now; I tried to apply the Ld formula to Attack in the yellow Troops Needed Calculator but it got stuck in an endless loop - troops needed depends upon Attack, Attack is effected by Leadership, Leadership penalty is determined by number of troops, number of troops depends upon Attack... yup, made me laugh too.

So test away and post your results this time! And don't screw with any cell that doesn't have red text cos I won't be happy and be aware, Google tells me where you live...