Here's the math I used.
60/40 = 60% of load pay, in this case $2, plus 100% fsc., in this case .40cpm. So 60% of $2 is $1.20 plus fsc = $1.60 to the fuel side and .80 to the non-fuel side.
40/20/40 I understood to be based on total pay, in this case $2.40. That breaks down to .96/.48/.96 which is $1.44 to the fuel paying side and .96 to the non-fuel side. Just like my correct Texas math showed.
We could make it even a little bit worse by going with your last suggestion of 50/50 then add 100% of the fsc which would be $1 plus .40 for $1.40 to the fuel side and $1 to the non-fuel side.
The bottom line is that 60/40 plus fsc is not the same as 40/20/40 of total pay. The former is giving all the fuel money to the side that should receive it. The latter is taking money that's not earned or deserved. IMHO and YMMV