Sample-drawing procedure: user-defined formula, nested parameters

hi, experts

     We faced one situation，use sample-drawing procedure:

          IF P2<= 3 THEN P2 ELSEIF P2 <=300 THEN  ROUND(SQRT(P2)+1) ELSE ROUND(SQRT(P2)/2+1)

     The formula was too long beyond 60 chars. So I was try to use user-defined formula, such as:

     But another question appeared:

     When I used TFKT:ZC() or TFKT:ZD() which used nested parameters, it couldnot correctly calculate the physical sample.

     Was I make some error for ZC/ZD?

     Any advice will be helpful!