Your weight, your general health, sleep problems, other meds you take, age, etc all strongly affect the amount of Adderall you need to correct (what you are looking to correct)
If you took Adderall IR 20mg, then you got 20mg; however, if you took Adderall XR 20mg then you only got 10mg. In a full grown human 10mg isn't going to do much of anything.
I now take 60mg and 80mg is where I should be but I have been taking it for about 8 years. 30 to 40mg was the right dose for several years. but I started losing words again and wasn't sleeping well again, the increased dosage fixed that. Now I need 60mg IR in the AM and another 20mg IRin the afternoon
Adderall is not for everyone that is older and tired and having age related memory problems BUT, for those problems it can be a wonder drug. Age, weight, other medications, and of course other medical problems are big factors in what your daily dosage should be.
And whether your taking XR or IR. The way that ALL XR meds are labeled as to the dosage they are is very confusing and I believe should be changed. For example if after several months of experimenting you find that 40mg a day is exactly the correct amount for you. If you are taking IR (Immediate Release) tablets then you are getting 40mg and it will last in your system for about 5 hours.
However, if you are taking XR (Extended Release) then you are only getting 20mg (immediately) and then another 20mg in around 4-5 hours. So the correct dose for you, is NOT 40mg it's 20mg. That's a big difference.
When testing to find the correct dosage I believe you should us the IR tablets so you know the exact amount of meds that corrects you problems without going overboard . Once you know how much your body needs then you can adjust the type (IR or XR) or the number of dosages per day to keep you optimized during your waking hours.
I prefer IR Tabs and when 40mg was the optimum dosage for me. I took 40mg when I woke in the morning and an additional 20mg at 2pm which carried me thru bed time.