The running theme here seems to be I cant measure 🙂 I walked the walls the service line would follow between the main and sub panels no less than twice. Took into account a foot of slack on each end, was generous around corners… Twice I came up with 50 feet. When I bought the wire (Alum 4-2-2-2) just to be safe, I purchased 51 feet…. Well, between my apparently metric tape measure and goofing on the sub panel end to the tune of a 6 inch cutting of the wrong wire to be shortened, I *almost* ended up short on the main panel end. Luckily the 100a breaker found home in the lower right side of the main panel and the ground line reached over to the left… barely.
But in the end, I have a fully functional sub panel feeding 7 circuits. Only had one out of 23 or so outlets wired wrong which seemed to upset a breaker (Did it on purpose to test the breaker, honest!), overall not bad for my first shot at fully wiring 3 rooms!