No, there is no "rule of thumb" or equation whereby you can figure, from whatever amount might be required by DL for a Skymiles booking on a particular flight, would be the "equivalent" amount required for a Flying Blue booking on the same flight.

The only rule-of-thumb that applies here is: there are almost never any DL flights available to book in Flying Blue, except where there is at least one transatlantic flight in the ticket.

Here's the results for a search for a one-way Economy redemption, followed by a search for a one-way Business redemption, on ATL-SAN - there is absolutely nothing avaialble to book in FB from now to the end of the booking horizon






If you're based in ATL, and want to be able to book Delta flights other than to Europe using frequent flyer miles, you should probably look at collecting DL Pesos, or just learn to live with the fact that your FB miles will not be convertible into any DL tickets for US or the Americas only.