I tend to stick with the 3-5-7 technique for the 166. At 3mph I go into notch 5, then at 8 - 10mph I go into notch 7. It's quite a sluggish DMU but it's physics feel authentic. I remember in the original GWE, the Class 166 had notoriously unrealistic physics and was like a rocket. These days its been altered feels very authentic to drive, so quite the transformation!
I've heard from somewhere (I think Dad Rail had a guest one time) that you go notch 4 to start, when the wheels start turning move to notch 6, then at 5mph go to notch 7. I believe something about a governer which caps the notch at 6 anyway below that speed was mentioned. I imagine it doesn't matter too much so 3-5-7 should be an acceptable driving style for other drivers not featured on Dad Rail. Found it! If you go to 49:06 it's mentioned Man I miss the old TSW2 HUD!