it becomes apparent that this can't really be solved by a formula (I don't think so anyway).
It can be, but it would be a program about 100 lines.
The concept I would try first is to make several formulas that calculate the offset of each face on the valve and seat compared to opening distance.
Then make another set of formula that does the same thing except that it would measure the distance from the edges of each face on the valve job.
Then make a set of conditional expressions that sort out the irrelevant measurments because at some low lifts, some measurements might not be useful.
Then make a function that finds the smallest value from all of the expressions listed above. I think that would be the curtain height, from that it is easy to calculate the area.
IN UG NX you can make a dynamic minimum distance measurment between two bodies, as you move the bodies the measument points move to what ever the current minimum points are. With a lilttle thought and effort you could probably output the results to a graph.
If someone wll post the dimensions of a good valve job I can do it when I get back from Japan.