We do the same, however we just do it before we push back from stand:

"Ladies and Gentleman, we are now ready for departure, so all mobile telephones and electronic devices should be switched off. If you are wearing headsets or earphones, please remove them now. For further information about the use of electronic devices onboard, please refer to the 'Essentials' section in the 'Highlife' magazine in the seat pocket in front of you. Please now ensure your seat belt is securely fastened and do make yourself comfortable. Thank you."

Usually at the point, on short haul, that the flight deck door is closed a minute or two before push-back.

When we get the 10/15 minutes to landing call:

"Ladies and Gentlemen, the Captain has turned on the fasten seat belt sign as we sill shortly be landing in XXX. Please return to your seat and ensure your seat belt is securely fastened, your tray table is folded away and your seat back is in the fully upright position with the arm rests down. All electronic devices should now be switched off and all headsets removed. I would like to remind you that mobile telephones must remain switched off till we have arrived at the terminal building and the engines have been shut down" etc...

So, we give clear cut off points.