A 50% win percentage would mean at least 57 points per season (plus however many for draws), which in the 20 team Premier League era we have achieved just 7 times, and the lowest we finished in those seasons was 7th, but typically was 6th. In fact, in that time, we have never even won 19 games in a season. If you add in the typical 6-10 points you'd expect from draws, that's a 63-67 point season, comfortably enough for 5th, very occasionally 4th at the top end. As much as I think that's where we should aim to be, it's insane to expect that of a manager who has spent most of his games as a newly promoted manager.
Even at 45%, that's 17 wins, something we've only done 5 times in the 20 team Premier League, and we got 16 last season.
Edit: Just looked up the last 50 years, we've only had 50% in the league 5 times in that time.