Normally, people use from 15-25. Professional player might use 25-30+. It depends on your skills and strings you are using.
hey, if you looking for better tension then better stick with the factory requirements i got one AT500 myself, usually string it with BG66 at 27lb(note:Brunei's system is still uncertain, so from the last time we managed to calculate out this value), it is roughly at that value for my AT and that was after two clashes which killed one high-end racquet(MP100) and a low-end gosen...
I use an armortec 500 and it is currently strung it at 21lbs with bg-65. i love this racquet can control all my nets and drops very well. however when i first got this racquet i used bg-68 ti at 23 lbs i found that the smashes were a lot better but my nets and drops were about the same.