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:
- Is there a clear rule made for a good reason?
- Does it make sense to use one punctuation symbol or another? Or none? Make sense = is my message going to be understood clearer?
- If I break an (abstract / arbitrary) rule, in favor of clarity, will it "look" very bad?
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.
You should do several of the things that he asked you to do, so that he doesn't blame you for his failure.