Rail (625Wh) XL Rail 5 dropper post upgrade - max insertion?

[markloch]

I'm building a bike out of leftover parts from my continuing Rail upgrades. I picked up a Sonder Frontier frame/fork, and am building a commuter/urban warrior/light trail rigid 29er bike, and at some point I may slap a bafang middrive on it, too. The seat post is the last item on the list (well, the fork, eventually) and wondering how long of a dropper I can install on the Rail?