What I did try was:
-Get the layer's origin (upper left corner) in the layout's center frame of reference (LayoutWidth/-2 , LayoutHeight/-2)
-Scale that position according to zoom (0.01*ZoomX*LayoutWidth/-2 , 0.01*ZoomY*LayoutHeight/-2)
-get the scaled position in the frame of reference of the layer ((0.01*ZoomX*LayoutWidth/-2)+(LayoutWidth/2) , (0.01*ZoomY*LayoutHeight/-2)+(LayoutHeight/2))
It worked somewhat, but it overshoots... There's a factor I'm missing on Construct's zoom . It seems that zooming doesn't ..... wait.
I might have it. Will edit this soon.
EDIT: Got it!!!
Since scroll rate is zero, it's zooming around the center of the SCREEN, not the layout. So scrolling rate affects zoom. Go figure. Replace LayoutWidth/Height with Display and it works