I am not sure where I got the idea that a comma must be added before "so that", but I have been doing this for a while now, and now I realized it might be ungrammatical, or is it? When would you add a comma?
For example:
You should do several of the things that he asked you to do so that he doesn't blame you for his failure.
You do not put a comma before "so that". Refer to this thread on wordreference, for example: https://forum.wordreference.com/threads/comma-before-so-that-conjunction-she-pulled-him-to-the-side-so-that.2974733/ Side note: The last two sentences seen in the first post are incorrect. A better way of writing those sentences are:
My pencil fell under my desk, so I couldn't see it.
George often told stories that weren't true, so no one believed him when he told about a deer in the school yard.
Commas go before the subordinate clause. "So that" pertains to a situation where someone does something in order to have an intended effect.
He told her a joke so that she could laugh.
I do agree that placing a comma in your example works for the sake of cohesion. However, it isn't correct, strictly speaking, in terms of grammar. It is up to you and whether you prioritize being a stickler for the oftentimes convoluted rules of the English language, or you prioritize your audience understanding you.
The coordinating conjunction "so that" in the second clause is normally used to show the purpose of the action in the first clause. But it can also be used to show the result, effect, or consequence of the action stated in the first clause.
You don't put a comma before "so that" or so if it is used to indicate the purpose of the action in the first clause, but you can put a comma before the so or so that if it is used to show the result or effect of the action just stated.
Look at the following examples taken from various dictionaries, where a comma has been used before the "so that" that shows the result of the action just stated in the first clause.
He chopped the wolf's head from its body, so that it immediately died (Wikipedia).
He got up very late, so that he missed the bus (The Free Dictionary).
The gravestones were covered with most, so that it was impossible to read the names on them (Longman).
The birds return every year around March, so that April is a good time to see them (Cambridge).
The word "that" is optional in these examples. You can drop it, without any difference in the meaning.
All languages have many rules; and they have also many exception, regarding grammar and punctuation. Sometimes the rules are necessary for obvious reasons, and other times they are just abstract conventions.
When judging how to write something, I follow the algorithm below:
I always try to follow the rules, but I keep in mind that, when communicating, the message that I want to convey is a lot more important than a rule made by some guy.
Also, I follow the algorithm above EVERY TIME when I write something, and need to decide what is better.
Going back your sentence, I had to read it several times in order to understand the sequence:
you to do so that he
Even if some rule tells that it is correct to NOT use a comma before "so that", I would gladly use a comma and break the rule (in this specific case), because it makes everything easier to read and understand - which is the ultimate purpose of the entire effort of communicating (writing).
Compare (the difference is one added comma in the second sentence):
You should do several of the things that he asked you to do so that he doesn't blame you for his failure.
and
You should do several of the things that he asked you to do, so that he doesn't blame you for his failure.
