Casual way of asking someone how they found or calculated a number? [closed]

Solution 1:

You could ask your colleague

How do you determine the flow rate?

and if you want to make it clear you are not trying to undermine them add

I seem to have made a mistake.

Please see Lexico

determine
_VERB

2 Ascertain or establish exactly by research or calculation.
the inquest is entrusted with the task of determining the cause of death

2.1 (Mathematics) Specify the value, position, or form of (a mathematical or geometrical object) uniquely.
Weil's work on polynomial equations led to questions on what properties of a geometric object can be determined purely algebraically.