Sir,
There will be no universal relationship that would cover all transducer/instrument combinations. Different transducers and different instruments and different combinations of the two will give varying results. Additionally, these things can change over time. A transducer's wear plate will become thinner with use.
I've never done this, but my instinct is that you could conceivably come up with a generalized table of equivalents using fixed transducer/instrument combinations. And it is certainly possible that once you complete a calibration with an IIW you can immediately see what response you get from the slot on the DSC.
However, if this regards a contract UT person coming into your shop for D1.1 Section 6 UT, that UT person should always have with him/her the IIW block. If not in their kit it needs to at least be in their vehicle so that they may refer to it if need be. If I were you and I had questions about a UT tech verifying cal off the DSC slot, I would make them show me that it works.
Mankenberg

I agree with kipmank. You will find about a 2 to 4 dB difference between the 2 blocks. IIW should be the standard and should be used to verify when in doubt. DSC should be a reference only since it is smaller and more portable. See the procedure being used for the inspection.