Too long a spoke, you risk pushing through the rim tape with the spoke end, or not having enough thread on the spoke to tension it properly. To short a spoke, you’ll not have enough thread in the nipple and it may fail.
I’ve used a spoke length calculator online before, sorry haven’t bookmarked it. The factors in spoke length are rim nipple seat diameter, flange size/spoke hole centre radius, dish, and spoke pattern employed. These may or may not be the right terms!
Note, the chamfer in the spoke holes of the flange is so the spoke elbow can avoid touching the flange and so avoid a stress riser.
Watch out for trueness in every plane, my first wheel was fine side to side but wobbled lots up and down. I gave up on wheelbuilding by the way…….
I’ve got this book, it is very good.
Professional Guide to Wheelbuilding.
I’ve used an old upturned bike with zip-tied on quick releases as a jig before. The rear triangle holds the wheel as normal, and the QR’s are attached to the frame so as the nut on the non-lever end overhangs the shaft, so it can be screwed backwards and forwards to interfere with the rim. Alternatively, use a steel frame and a clock gauge.