Window reveal - Place window on specific place

[Zendrix]

Hi all! just starting to mess with Archicad.

My question is regarding placing a window:

I have a stone house which existing walls are 50cm and made of granite. I am trying to draw the existing situation, but having troubles placing the window on the narrower part of reveal. I messed around with settings, but cant seem to move the window to the right place. 

I managed to design the reveal correctly, but missing pushing the window to the outter end of it.

Please see attached images.

Thanks

Operating system used: Windows