Thanks for contacting us. Where accurate placement like this is important I recommend that you don't use the generator's Extend X/Y Size to Area
mode. Instead you can use the Generator's XY Size
parameters to create a patch of floor that's large enough to cover the area and then move and rotate this into position using Max's transform tools. You can then just use the clipping spline as a final step to cut the correct shape, but it won't influence the position or rotation and you can line it up how you like. I've attached an example that uses this idea.
I hope that helps, but if you have any further questions, please let me know.