Measuring a spider for a rim
The first thing you want to do is make sure the spider is even on all four spokes, and has the proper "curve". Set the spider on a flat surface and measure the distance from that surface to the tips of the spokes. theses should all be the same height.
When all four spokes are even, and the curves look good, the spider should sit flat.
Once the spider is nice and flat, it's time to measure. As you can see from the pictures below, place the scale so that it starts and ends at the inside edge of the spoke ends. In this case, the spider measures 12 1/4".
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Now that we have the spider measured, the rims inside diameter will be made to that dimension. (in this case 12 1/4") Now the rim should fit the spider just right. Fine tuning may be necessary on the spider to get a perfect fit. I rarely see a perfect spider. But hey, they are 100 years old.
Hopefully, this information was helpful and leads you to a beautifully restored steering wheel.