That is a tricky alignment - you really have about 4 angles coming together there - the vertical side, the angle of the side as it goes toward the top, the flat writing surface of the desk, and the drop front. I made a mock up of the drop front out of cheap pine - just a rectangle of 1 x 4's. I cut the mortises for the hinges in the edges of the mocked up pine and then cut it down slowly to the proper size. The height of the 2 vertical pieces that are between the drawer and the sliding supports is the other critical piece. I started with that piece being a bit long, then trimmed it back as I adjusted the size of my mocked-up drop front. I also made the sides of my desk a bit tall to start, and one of the final fitting steps was to use a block plane and take the slanted sides down to the same plane as the drop front. Once I got the fit I wanted with the mocked up pine rectangle drop front, I used those dimensions to make the real one.
I hope that helps some - it is a bit confusing to read, but maybe that will give you some ideas.
btw - visited Vancouver a few years ago and really enjoyed it.