This is not a restaurant.  The tip shouldn't be based on the house fee, and if it is, you should tip more when the house fee is so cheap.  When I used my waitress analogy, I simply meant to show my baseline tip, and then how I adjust for quality,

First of all, not all massage places charge the same.  High end "all american" places usually charge between 180 and 220/hour.  I stick by my same rule there.  $40 for standard finish.  I am not going to pay more to the house and also tip more.  That's why using a percentage of the house charge doesn't work.

By definition, tipping is not required at all, but you should do right by the girl, especially if you plan on ever going back.

If you see an independent girl, she set's her own price, and tipping is not necessary.  But at an agency, that's not the case.  I don't know specifically, but I'd be surprised if most agency girls got even half of the fee.  And that's not even talking about AMPS.  That's a brothel where you are paying more than an AMP.  At an AMP I would be surprised if those girls got more than $25 per massage.  Or maybe they work only for tips.  That's possible too.  So, do you want to be the guy who makes the girl work and then gives a minimal tip?

You are welcome to be cheap like that, if you like.  You can also be the one who leaves a 5% tip at a restaurant.  But I wouldn't be able to look at myself in the mirror if I treated people like that.