Hi,

For many reasons, MIR relies on Ambisonics, which is based on a so-called coincident microphone array of four capsules. A typical Decca-tree is a spaced setup of three omnispheric microphones, so there is no direct aequivalent, especially as there is not "the" Decca-tree.

Personally, I would start with the "Wide X/Y Stereo Hypercardoid" setup and add a bit of distance between the two decoded capsules (values between 10 to 20 are a good starting point). You might also try the Blumlein "Crossed Figure-8" setup if you aim for more reflections from the back of a venue. Again, you might try to add a virtual distance to the decoded mics.

HTH,