What is the minimum height needed between the top of the air handler and the supply duct?

We do a lot of replacement of electric furnaces which have about 10-15" between the top of the air handler and where the supply duct starts after the plenum.

The issue is we need to raise the newer furnaces about 13" so they can take their air from the bottom, which raises the furnace height substantially.

In some cases it simply won't fit, and we need to put the air handler in 2 piece (coil in the return).

Is there some rule of thumb for minimum or ideal distance between the top of the furnace and the supply duct? (for proper air mixing, minimal turbulence, air flow sound, etc.)

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