Take off finishes at 35 feet and the initial climb starts.

There is a reference here

http://www.intlaviationstandards.org...efinitions.pdf

However, the word 'takeoff' when used for electronic devices is a bit of a misnomer, as they should be off for critical phases of flight and I'm sure that's rather higher than 35 feet.

The logic behind the electronics ban during critical phases is to avoid any disturbances which may not be recoverable.

I've held a PPL for 20 years and I always switch off my electronic devices and keep them off until either the belt sign goes off or the houses look very small, as I saw one of my nav instruments develop a 30 degree error, when a pax accidentally switched their Nokia on.

Better safe than sorry, even though I believe modern aircraft are well protected.