I myself have had the same question since I started back in September of this year. My friend made Amateur after he finally shot a score of 87 and I did a few days ago at 89.4
So it should be under a score of 90 and also found out that your tier will not go back to hack if you happen to shoot above this tier level. Which is good for me since I finally was able to purchase and create a Country Club for me and my closest friends to play and socialize on. Incidentally I played for 14 consecutive days and I believe this practice and the XP experience I learned helped me accomplish this as well.
Oh yeah here's another tip
Always invite your fellow golfers in your friends list to challenge your score on stroke play and above this helps greatly. you get 10 XP to send the challenge BUT recieve 50XP if they accept it and play
Hope this helps as much as it did me & happy golfing
Joe