HI'er, I think it's obvious he is talking about short strips, in which case 70% at half way would be a very good, practical performance indicator. When it comes to 30 year old light aircraft, P-charts may not be worth the paper they're printed on - they are deffinately only a guide, there's just too much variation in performance from aircraft to aircraft.
All though I do agree that in anything a bit bigger, performance does need to be calculated more accurately. Please don't automatically knock people for applying a bit of common sense, there are far too many pilots who lack it. There will come a point when you reach the limit of 'the book' and will have to save your own ass.