Not to beat a dead horse but yes trial and error.
When I was a child my parents dealt me different doesages and medicines like a card dealer until they found what they thoug was best for me.
Currently I've only been on Adderall for two days for the first time in 10 years (long story) but the first day I took 15mg of addy IR and 4 hours later took 15 more, which is what I was perceived due to the fact that I was fat and my doctor wasnt sure if 15mg would be sufficient. Anyway it was great. I didn't really notice any of the feelings of being "high" like euphoria and stuff so I wasn't even sure it was working but I found my attention span and focusing abilities were through the roof. Also I felt calmer and my thoughts were slower.
However today I split my 30mg pull in half and took 15,g in the morning. Then 4.5 hours laters without thinking I popped another single 30mg.
I can easily say that it was too much. It felt really easy to stay focused on one task compared to not being on the meds, but unlike the day before I physically felt the medications. I can't explain it but I could sense them taking over my body. And yesterday I didn't really feel the meds stop working. I just kind of realized one that Iwas felt more or less normal, (like I cought myself talking louder as I usually do without meds)
But today just like I said I felt the second 30mg dosage really strong on the come up, earlier this evening I just felt relaxeded one second and then I could suddenly notice everything around. The guy on tv had hair that reminded me exactly of my dad, which caused me to think of my mom, then their dog, then our dog, then how she is going to love our new house, then feeling anxious to move to our new house (my fiancé and I Porsches our first house and we move in next month). Then I just remember thinking to myself, "well I guess they stopped working". I also felt groggy very quickly after that. Needless to say I'm sticking to the two 15s a day for a while lol.