If 100% goes over the number of attackers, slightly lower settings (in this case as low as 67%) will still go over the number of attackers (your formula, using 8 attackers, 75% luck): "8 * 0.60 * (1 + 0.75) = 8.4". However, as I showed on the previous page, the maximum is actually 7.
I don't so much mind implicitly capping at the number of armies, my problem was that it's a clear sign your formula must be wrong. Same thing with assuming standard kill rates: if it doesn't work for non-standard kill rates, it's actually rather unlikely to work for standard kill rates.
Correct formula:
Killrate + Luck * ( 1 - Killrate )
In light of your statement quoted below I'll forego explaining the colour-coding.
I make my statements giving sufficient credit to my audience to make the most basic of assumptions. Apparently I should change that for your sake...
Before typing such a statement, you might want to double-check being right yourself. Then, after making sure you indeed are, you should still phrase it more politely.
To your credit, your formula for minimum number of armies killed:
( 1 - Luck ) * Killrate
does seem to check out.