No, there's no subtraction involved when it comes to interpreting ST/LT fuel trim. Your ST number reflects how much fuel is being added/taken away this very second to meet the pre-defined ideal ratio (14.7 parts of air to 1 part of fuel), your LT fuel trim reflects how much fuel has been added or taken away over a given period of time to meet the ideal ratio. Positive numbers indicate that the computer is adding fuel (lean condition), negative numbers indicate that fuel is being taken away (rich condition).
Your fuel trim #s could be indicative of a slight vacuum leak; while at idle the 02 sensors are reporting more air in the exhaust than the PCM wants, so the PCM compensates by adding fuel. When the engine is revved to 2500 the extra air moving through the engine kind of masks the vacuum leak, and the fuel trim #s come back closer to zero/ideal. +10% isn't super high, but in my experience single digits in either direction are what you want to see.
Here's some good reading on fuel trims if you want to get more into it from one of my favorite automotive sites:
https://www.aa1car.com/library/what_is_fuel_trim.htm
No, I use OBD fusion because it has more parameter IDs than the standard CARB stuff, so I can look at manufacturer-specific parameters if I ever need to.