Thank you for the replies, gents. Very much appreciated.
Yes, I did think 40-50% was more typical than 70%!
Denti: am I correct in interpreting the procedure you set out to mean that you would potentially hold the aircraft with brakes on with your final T/O power setting for 30 seconds? If so, crikey! That's about the same length of time as the take-off roll itself! Must be quite something to experience. Would you brief the passengers before push-back that you're going to do that?
Dariuszw: sod off out of this thread.