Maybe it's too late, but I had the same problem and just found the solution today. The "Alt" function, you can add as many formulas as you want, the result is the first one (from left to right) that return a valid numeric value.
Would return 0, because of the division by zero returns a dash "-"
I do not know such a function.
If you want to reuse several times a function, you may use the preload technique. You will have 2 consecutive LOAD statements, and only the first one will populate the table. Take care, the LOAD which populate the table is the 1st one, and the second load is the one that will be source of the 1st !!
LOAD if(isnull(A/B), A, B) as A2,
xxxx; (no FROM the source is the second load below !!)
LOAD MyveryDifficultFunction as A,
MYsecondDifficultFunction as B,
C, D, E ....