I agree that finishing outside of the top 7 would be pretty poor but I'll give him a bit more leeway and say top half, especially if we don't bring in anymore players. I just don't think the board is going to cut him loose unless we show absolutely no ability this season and, while outside of the top 7 is poor, a bottom half is just unacceptable.
If you think of it from the angle of the results it would require to finish in those positions, rather the actual position, that's where I think eighth would be untenable. Think of all the matches we screwed up in last year, the two points out of a possible 24 we picked up against the top four, the 45% count of goals conceded due to own goal or error - and we somehow still managed to pull sixth. Liverpool surprised most, Everton showed stunning improvement in one year under a new coach, and we still finished sixth.
It would take something drastic for us to finish outside of the top seven with this squad, almost a concerted effort of sabotage. So I don't think it's the position specifically that would do him in, but rather the sheer quantity of shocking results that would do so. If de Boer is still at Ajax at that point, and we do that poorly with this squad, I don't think Poch has much of a chance.