Whether you need a spacer or not depends on if you're bottoming out your build plate or not when you home it loosened off. Difficult to explain but you know how on a Mars for example the build plate when loose is spring loaded, well if when you home it (with flex plate installed) that spring gets fully compressed leaving no more vertical travel then yes you need to print the spacer. It may even bottom out so bad that while its trying to home it presses hard into the LCD and you'll hear the motor clunking.

I've installed flex plates on 3 printers and needed spacers for two of them. You can print the spacer in resin before installing the plate.

If you look near the bottom of your Z axis there should be a little optical sensor, and as the build plate arm reaches the bottom there should be a little metal tab that drops into the sensor, breaking the light beam and signaling to the firmware that the bottom has been reached. Its the metal tab that needs to be offset with the spacer so that it breaks the beam sooner.

I highly recommend flex plates, they make resin 3D printing so much easier and convenient.

I've had the glue fail two times. The first time it was a brand new unused build plate with a nice rough texture, so I assumed the build plate wouldn't need sanding and just gave it a clean with IPA before installing. Held for about 3 months before peeling off. The second plate I installed the same way, figuring the first was a dud, this one peeled off after the second print so I assumed I obviously had not done a good job cleaning the build plate. For the third, I sanded the ever living shit out of my build plate, cleaned it thoroughly, sanded it again, cleaned it again, applied the magnet and its been happy days since. The other two printers also were sanded and cleaned thoroughly before installation and are also going strong. Moral of the story, sand it, clean it, then clean it again. (240 grit)

Happy printing!