check box not center Xaml

I created a radio button in a custom window with a custom style, the radio button sits in a grid panel. When I check the radio button and the window is not maximize everything looks fine but when i maximize the window the radio button optionMark is nor centered.

I tried to remove the style and return to the default style but that did not help. i also tried to set "snap to grid" and "Use Layout Rounding" to true. "Use Layout Rounding" made it worst and "snap to grid" didn't do anything.

Thanks for the help Roi