S=luminosity (or flux, either consistently used, works) of S
L=luminosity (or flux) of L

-2.5*log10(L+S)=-2.5*log10(L) -.2
log10(1+S/L)=.2/2.5
S/L=10^(.2/2.5)-1

* ? 10^(.2/2.5)-1
    .2022644346174129    

Let a=fraction of L NOT covered

-2.5*log10(L+S)=-2.5*log10(S+a L) -1
log10((1+S/L)/(S/L+a))=1/2.5
(1+S/L)/(S/L+a)=10^(1/2.5) ...call that Q

1+S/L=Q(S/L+a)
(1+S/L)/Q-S/L=a

* ? (1+res)/10^(1/2.5)-res
    .2763656577052254    
* ? 1-res
    .7236343422947746    !fraction covered