And put the assembly in the bike.
Adjust for length. I tried to get about the same amount of post inserted in each stage, but as long as you have a good minimum insertion in each clamp, you'll be fine.
My consulting structural engineer (my son, also a Twenty owner) says that the design of the Twenty frame is plenty strong enough to deal with a longer post. Well, presuming you aren't too far into the Clydesdale Class that is. If you are a bigger person, I'd suggest you investigate a strut of some sort.
If you loosen both clamps, the seat will still collapse all the way into the frame. And if you ever want to remove the telescoping post, the original seatpost can still be returned to normal use by removing the extra bits and returning it to its right-side up position. With what you trimmed and the notch, it'll effectively be an inch shorter, but turn the slot to the back and it will be quite useable for all but the taller riders.