There's a long thread about probing formations on Fail Heap Challenge here. But to summarise :
Optimal depends on your circumstances.
If signal strength is your target, and time/effort setting up your scanning configuration doesn't matter, then the best layout would probably be either a cube or rectangular antiprism. These two would probably take the longest to set up out of the different kind of arrangements but might be good for scanning down a ship that has been purposely fitted to make scanning a lot harder, previously known as "unprobeable" set-ups.
Usually an Octahedron is a "good enough" balance between ease of set-up and scan strength. It is reasonably easy to make by imagining a compass laying flat where your ship is and launching 6 probes, then drag one of them in each of the directions North, South, East, West, and also one straight up and one straight down. Handily, each of these directions to drag in correspond to the same directions as the positioning arrows around each of the probes.
If time is a telling factor (which is most likely in combat situations) then the regular tetrahedron still works fine, although you may get slighty less scan strength than you did before.