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The John Armstrong book, Tracklaying for Realistic Operation, lays it out better than ever. He has a table that tells you the closure rail radius for each frog Number. This would be what you would go by to keep trackage up to the minimum standard radius you set for your curves. The closure rail is the rail between the end of the points and the frog. Track through the frog is straight. This is what throws people off, so he also gives a "substitution radius" or the radius the switch will replace as a whole unit. The frog number is the angle of the two tracks where they cross. And as stated above it can vary from manufacturer to manufacturer. Atlas even has mislabeled a #4 1/2 as a #4. In short, you need to have the switch on hand to determine how it fits in a curve.
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