@Shrunken_Head_Pedaler
Use a ruler to measure the following in mm. If you have a vernier caliper great, if not a good ruler will suffice.
Use thick paper such as a mail flyer for cutting out around the axles where you can't stretch across a ruler easily and then measure the distance between the paper for the flange diameter listed below.
Find the spoke length of anyone of the spokes you removed.
How to
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Find the spoke flange diameter of the original hub. The diameter is measured center of the spoke holes, not the outside of the flange.
How to
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Lastly, find the flange width of the original hub. The measurement points are taken center of both flanges which is around several mm thick.
How to.
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I know the wheel is a low profile ISO: 559 hoop, and I already have all the Shimano 3 speed hub data.
I can calculate the rim's ERD with the measurement you provide and tell you if the spokes can be reused without re-threading. You'll need to know this info for spoke calc anyway. You're not making a offset wheel which makes calculating easier.
Gluck