Plane

Can someone please tell me why I cannot get accurate values for finding if a point lies in a plane. I have this setup so far a simple grid with the axis like this Y/ /Z / / // --------X I Create square primitives on the X and Z plane as length and width. Height is measured in the Y plane. the tiles look like this 0--1 Two polygons forming my rectangle / // // / 2--3 Now i have a function that tells me exactly what side, or what poly im in- in either poly 0,1,2 or poly 1,3,2 Then i take the three known points for the poly that im in lets say 0,1,2 ''note the whole reason for this is that I always know my x and z locations but am trying to find where my Y value should be to place me on the plane or poly im in. at this point im using the plane equation but it isnt comming out correctly. can someone maybe give an example of how to find for Y in the plane equation considering the above axis orientation ?