The OP says, "I had read explanations that said that 'may' should be used after 'so that'". I don't think there is such a hard and fast grammar rule.
Can and may can be used to express possibility and permission with some differences.
Possibility: (from English Grammar Today, Cambridge Dictionary)
When we talk about possibility, we use can, could and may,
but they are different in meaning.
It can be dangerous to cycle in the city. (This expresses what the speaker believes is a general truth or known fact, or a strong
It could/may be dangerous to cycle in the city. (This does not express a general truth. The speaker is only expressing a weak
We use can, could and may to ask for permission. We use
can and may, but not could, to give permission. May is less common:
Can I ask you a question? ( informal )
Could I use your phone? (more formal/polite)
May I use your phone? (even more formal/polite)
Also from Oxford Living Dictionaries:
‘Can’ or ‘may’?
People are often uncertain about whether there is any difference
between can and may when these verbs are used to ask for or grant
permission. For example, is one of these two sentences ‘more correct’
than the other?
Can I ask you a few questions?
May I ask you a few questions?
There is a widespread view that using can to ask for permission is
wrong and that it should only be used in expressions to do with
ability or capability, e.g.:
Can she swim?
Can you speak Italian?
But the 'permission' use of can is not in fact incorrect in standard
English. The only difference between the two verbs is that one is more
polite than the other. In informal contexts it’s perfectly
acceptable to use can; in formal situations it would be better to