I’ve held off answering your post in hopes that someone up here knew of a quick way to determine pendulum length for Sessions. Short of calculating the train I can’t think of any way other than the lazy way I do it. When in doubt I usually just hang different length pendulums and check the timer until I get close. There may be a quicker way like different escape wheel teeth count. Maybe someone here has the answer or has a few Sessions movements laying around they can check.
Sorry I couldn’t offer more help John.