Your working should look something like this.
20-(5x3)=5-(5x1.8)=-4+(3x1.8)=1.4+(7x3)=22.4
It is important to remember that the SALR is not constant as we gain altitude. Latent heat is released as condensation occurs and the SALR value actually increases and eventually could equal the DALR
We can debate this till the cows come home but at the end of the day the CAA say that the SALR is 1.8 degrees per thousand feet.
Your thinking was perfectly correct. 1.8 is the number to remember.