Shoot twice, cut once
If you are struggling to get the whole image sharp, take two images, one focused on the building and the other on the perspex. Open the images in Photoshop, drag the perspex-focused image onto the building image and line them up using the Move tool. With the Marquee tool set to rectangle (alt/ctrl + M), draw around the reflection. Make sure that the rectangle is level along the horizon, then click the Add vector mask icon at the bottom of the layers panel.