First, if you haven't tried it - a high protein breakfast will make the stim meds more effective.
Second, those are called recommendations, because that is all that they are. All kids are different. You start low and slowly increase until it works. If you need 70 mgs, then that s what you need.
However, sooner or later you may want to experiment with the Adderall type of medications.
The horror stories of meds are usually because the doctor did not go low and go slow or the parents did not communicate to the doctor what was going on, or the doctor did not communicate to the parents what to expect.
There would be no harm of using the lower dose on the weekend.
The standard for effectiveness of a med is time of duration. If it does not last as long as it should, then its time for an increase or a new med.
Hope this helps.