Hi Brian and welcome to the forum,
Sorry for not responding to this sooner but have been mostly away from the internet the past couple of days.
Some modern clock movements do stamp the length of the pendulum on them. You can usually buy the complete pendulum for these from your supplier by specifying the type of movement ie: Kieninger, Hermle. If missing, I often used old catalogs and advertisements to see what style of pendulum to use. Often times suppliers such as timesavers etc. can help determine what pendulum or bob to use by describing the movement and clock to them. The same movement may have been used in many different cases so pendulum styles differed but weight and length were the same. If a pendulum is too heavy it can stop the clock as the movement is designed to push a certain weight. Takes more energy to push a heavier object.
Your trial and error method is usually what most of us resort to when trying to figure out the weight as it’s next to impossible (well not really but it’s really tough) to calculate what weight your movement will need. I think you’re right on track Brian.