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For my government work I have been trying to work out spacing
precisely.

I am correct in the following:

If A and B are paragraphs, occurring sequentially, then the space
between the bottom of A (to descender) and the top of B (ascender) is

The leading of A + the maximum of (A.After and B.Before Spacing)

In other words the before and after spacing are not added together,
the spacing is based upon the larger of the two values.