Use your published data. It will provide a rotation speed. That's where you rotate.
I couldn't agree more in principle, though you might want to consider how that sits with you apparent assertion that rotation below Vmca is unwise. Here's the published data for a PA30B Twin Comanche from the Piper Aircraft Corporation POH:
Vmc
90 mph
Vy 112 mph
Vx 90 mph
Vyse 105 mph
Vxse is not published but with a wopping 260 fpm OEO at Vyse, I doubt it's going to be much less than 105 mph.
Performance data is published for a lift off speed of
80 mph attaining 91 mph at 50 ft.
Here's the manufacturer's description of a short field take-off:
"If the airplane is off the runway at less than 90 mph, it is essential to fly level a few feet off the runway immediately after lift-off until reaching 90 mph... Since the airplane cannot be controlled in the event of the sudden power loss in one engine, be ready to reduce power promptly."
It really is possible on some light twins to lose the engine, accelerate to Vxse or Vyse and climb while you bank into the good engine, clean up, and return to land.
On
some, yes, I'm sure it is. On others, like the PA30B, I think I'll stick to committing at Vyse thanks very much.