Finally found a reference to it but still....bloody vague!
• Decision speed (V1) is faster than engine failure speed (VEF) by the
demonstrated pilot engine-failure recognition time, plus one-second.
• If continuing take-off:
- Rotation speed (VR) must be faster than 1.05 VMCG
- Speed at 35 feet altitude (V2) must be faster than 1.2 VS
- Speed at 50 feet altitude (V50) is considerably faster than 1.2 VS
Ref:
http://www.enhanced.aero/pdf/performance_raisbeck.pdf
So, maybe Venr?