I think you misunderstood, the %K is amount of percent of money you will lose (risk per trade) not a %of allocation to invest

the kelly criterion derive from log return of investing

E = P*ln(1+R%K) + Q*ln(1-%K)

dE/d%K = 0 ; you get kelly formula which is:

%K = %W-%L/R

E is expected return
P is times you win
Q is times you lose
R is ratio of win/loss in your case 20%/20% is 1
%K is % of money you will lose each time

and R is consider using ratio not actual percent of loss because
the loss is your %K
and the winning is you R*%K