Jointer and hand plane technique are pretty nuanced in my experience. While planing a face of a board flat is straightforward, I find that making a straight and square edge is easy to screw up. With a hand plane it's easy to get the edge out of 90 degrees while focusing on getting the edge straight and vice-versa. A jointer with a fence does a much better job of keeping the edge square.
You can't just run long edges over even a normal-sized jointer and expect the tool to automatically do what you want. To get straight edges you really need to assess each edge prior to jointing and after each cut as well. If the edge is bowed more than a little, it's often smart in the shop to band saw a straight line and go to the jointer after. To avoid tapering a board in a quest for a straight edge, plan where material needs to be removed first, usually from each end or from the middle. It can help to put marks on the board face where you need to remove stock first. If the edge is bowed with the ends needing correction, a common situation with a board perhaps 4 ft. long, you might need to joint off 18" from each end and then reassess the board. This approach leaves "steps" on the edge. Once the edge is jointed with enough steps that you can lay a straight edge on it and have the distance between points of contact shorter than your jointer outfeed table, then you're ready to make a full length jointing cut to produce the finished edge. Obviously the outfeed table on the Festool isn't very long so jointing a truly straight edge will require some fussiness. It's roughly comparable to edge-jointing with a jack plane. If I'm hand-jointing an edge with a hand plane, if it's longer than about 24" I'll prefer a longer plane than a jack.
I use a 78" level to assess boards while milling. It can be balanced on the board edge to look for light.
