Folks,
Our normal take-off planning is based on worst case, ie an engine failure at Vef. If the engine fails later, we are much better off but have no way of assessing just how well off - worst case planning remains our best protection.
If it appears likely that anti-ice will be required before the take-off is complete, then worst case planning would naturally lead to calculating the take-off as an anti-ice ON take-off, regardless of when you actually select it.
The take-off is complete when the flaps are up and climb power is set, usually above 1500' AAL or higher if terrain so dictates.
Stay Alive,