Percent based lists worked quite ok in 2nd edition and I have to say I never liked the FOC and GW also seems to hate it.
You have so many choices which are "You know this is an HQ choice, but somehow it isnt... you may purchase a number of them but they dont really count..." as well as Formations (lets ignore the FOC) and allies that do not really count as allies but expand your FOC.
This is a gigantic mess. If I had to guess they probably just reintroduce the 2nd edition army comp.

0-50% HQ (could be less but who knows)
25%+ Troops
0-25% Elite
0-25% Fast
0-25% Heavy
0-25% Allies (usually was tied to a restriction e.g.: If you chose allies you couldnt choose heavy support)

It was a little bit easier in 2nd as you had individual lists and no "Elite" or "Fast". There where just Squads (everything), Characters (HQ, including squadleaders) and Support (Tanks and Walkers)
So a typical Chaos army "FOC" was:
0-50% Characters (including every character + equipment)
25% + Squads (including everything from Terminators to Bikes)
0-25% Support (everything from Rhino to Landraider - including Allies!)
0-25% Daemons (everything from Bloodthirster to Nurgling)