I am a big fan of experimentation to optimize your results.

If you don't have much time available to work this spring, maybe lay down some landscape fabric and try 1-2 beds at 18", and then 3', and then 4.5'.

If your water slowly clears up enough this spring that you can observe the deeper nests, let the BG "tell" you which depth they prefer for your particular pond conditions.

You can then do more of those for next year's spawn.