For one, the custom mask is already very frustrating to use, not the least of the reasons being that I can't edit it afterwards and there doesn't seem to be a zoom for any precision. For two, if an object happens to have a quarter circle in it or something along those lines, what do I do about that? Actual games apparently don't have any difficulty doing these things, so why should construct?
If you want more precision while placing the points, hold down the control button and use the scroll wheel to zoom into the layout. That will allow you to place them more precisely.