I don't know how you set up your spokes, but try this if it's different that what you did.
Get 2 300mm or longer spokes and 2 nipples. Measure each spoke to exactly 300mm (you can use 250, or 200mm) and cut off a bit long and grind it back to 300mm. tip the spokes in glue and rhread each into a nipple until it ends between the bottom of the slot and top of the nipple (or 1/2mm short of the top). Let it dry.
Now you have gauge spokes that each measure 300mm from the ERD, and you can run them in from opposite holes, and add the gap, or subtract the overlap from 600mm (or double your gauge length) for a true ERD that includes the nipple head thickness.
I suspect that your problem is that you measured only the exposed spoke, and so your readings were short by the length of the nipples.
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FB
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