For starters, you're not supposed to be "guessing" the sum but rather "telling" them how much they have and when done right, you will seemingly be off by a dime, only for that dime to be found in the opposite pocket, a back pocket, etc.
Secondly, why are you telling folks that it's just tricks? Not only does this demean what Mentalists do (and hurts your fellow performer) it robs you of the optimum strengths the craft hosts. It's ok to claim Psychological Influence if you must be trendy, but don't refer to it as "trickery" the more you do this the more folks will see you as a magician vs. someone "special" be it one studied in Psychology, Body Reading or born with unique gifts... our job as Mentalists is to validate the claim and make it believable, not do play trained pooch and jump through hoops on demand.
I promise you, if you change how you observe and present such things in your mind -- distancing yourself from magic and being a magician -- you will see a stronger sense of bewilderment in your audiences and even in how trained or schooled people within Mentalism see your work. It's subtle but it is akin to how we treat our PARTICIPANTS in a routine vs. seeing them as SPECTATORS; we work with them differently and make them an ally rather than treating them in the 3rd person. Linguistics and how we use words & terms affect us and our actions just as they do those observing or participating in our demonstrations.