I've found 1 meter below keel is about the optimal depth. Any deeper and it may not trigger. Any shallower and the risk increases that the torpedo will hit the hull at its most rounded part, deflecting it downwards. At that angle, chances are high the contact pistol wouldn't trigger.
I could sink a Revenge class with 3 eels, 2 of which magnetic set 1m below keel. So, I'd consider this pretty efficient
You may choose against magnetic in rough weather though: the target might be going up and down due to the waves and it's hard to gauge the right depth setting for your torpedo. In such weather, impact pistol at a depth about half of the target's draft usually does the trick.