I was looking for some useful information to help answer your question, and ran across this nugget in "Principles of Naval Weapons Systems" by David R. Frieden:
"The [submarine's] optimum approach for close approach to a target with minimum probability of counter-detection is approximately
Best Depth = 17 x SQRT(Z)
where Z is the layer depth in meters.
This is accurate down to a layer depth of 60 meters. Below that, the best depth for approach is a constant 60 meters below layer depth."
I doubt that this holds true for SH4 (unless they've modeled sonar layers with more fidelity than I believe), but it's interesting none-the-less.
