Thank you.
@Elektron:
float is a versatile type; but I don't think I'll count on it to use it for all the range it is capable of in the same variable. In situations where precision is crucial, like coords I was thinking of reducing the accuracy, in terms described at
wikipedia - accuracy vs. precision
and snap the values (round them).

@Joey:
d means precision in no of decimals, right? It takes me some time to digest it Please say which one is true: A or B:
Supposing both of the operands are of type var
A. Either you multiply or divide, the inaccuracy of the result is the same and raises with no of decimals of the operands. Or
B. The result of a multiplication of any operands has a greater accuracy than the division of the same operands.
From what I understood I tend to say that A is true. In the formula you wrote first, dz doesn't depend on a type of operation between x and y.