Since I really dislike having to scale/position the Astropad active area to just a portion of my Mac screen, I've used a utility called "SwitchResX" to create a custom Mac screen resolution that matches the dimensions of my iPad screen.
Since I'm using an iPad Pro (and a non-Retina MBP), that means I needed the screen to be 1366 x 1024 pixels. Or exactly half the resolution (and one-quarter the pixels) of my iPP screen.
Presto! The iPP now maps perfectly to my MBP's screen (at half the resolution).
SwitchResX isn't the easiest program to use, and adding a custom resolution requires you to reboot into your recovery partition to disable (and later re-enable) El-Cap's "System Integrity Protection" with a terminal command. But if you're not afraid to muck around a bit with the technical stuff, then you can make your Mac and your iPad screens match up much better without scaling/interpolation introducing drawing issues.
One tip through: When you create your custom resolution in SwitchResX, COPY an existing resolution (such as 1280x1024, and modify it instead of starting from scratch. Because if you get the display timing wrong you'll have to reboot and reset your PRAM (Option+P) to get out of your defective screen mode.
Hope this helps a few people.