How to create an accurate mask for a UV 3D resin DLP printer
I've created software to do just that. See the project on Github: CreateMask. It has a wiki page that explains what to do.

To summarize: you measure the LCD build area using a multi-meter and a light dependent resistor. You do this with low and high intensity masks. You feed the numbers in CSV files to the software, and the software will generate a mask for you by polynomial curve fitting. See this page that explains in detail how the mask is created, if you are interested.

You can download a release here.