[VB.NET] mouse click in external window

hello,

i managed to create a program to perform mouse clicks of all types using win API..
calculate the original mouse position and add a few some maths for the new X,Y
left click down\up right click down\up...etc.
using hotkeys with KBhook.


but, all of that only applies to the main screen in general.

what im looking for is, a way to focus on a single window, and do the maths inside that window only.
knowing that the window size is fixed, and it can be moved.

so if i move the window from corner to another, i want to click the same position within that window, despite the window location.

any help would be appreciated. and thanks in advance.