How to map Z to Brep V

Hi
Assuming an open Brep which V domain is always incremental in z axis, please help me figure out fast enough mapping of Z to V domain.
I need to find the V value of Brep surface domain when z is equal to 48 mm to be feed at Isotrim/Isocurve for example.
my try using brep|plane was so slow in a heavy brep case.
Thanks