97% or greater effectiveness
The 97% figure is based on the Typical Use column from this wikipedia page, assuming that a condom does not negatively interact with a diaphragm and spermicide. This is, in almost all cases, a very reasonable assumption.
Math: Male condom, 15% fail rate. Diaphragm + spermicide, 16% fail rate. 1 - .15*.16 = .976, or 97.6%.
If perfect use is assumed, instead of typical use, the figure rises to 99.9% effectiveness. (1 - .02*.06 = .9988)