Hi all, I have a wall-hanging porcelain time-only wall clock with a german movement of unknown, but probably less-than-40-years-old make. It needs a mainspring, it's a smaller 3/8' wide spring. I only have the last few turns that were left on the arbor, the rest is gone. It's an 8-day movement, and I'd like to get a spring for it. Looking at Timesaver's 8-day springs, they are all 96" long. That seems a bit large to me. Is there a way to calculate the length of spring I need? I can get pics etc if needed, I realize my description fits about a gazillion or more clocks.