If someone asks me, "Shall I start?" then my responses would be "Yes, please" or "If you would." There's not a technically simple answer, because "Yes, you shall" suggests a strong obligation, and "Yes, you should" is a suggestion. Shall is a funny modal to juggle around, but very useful once you get the hang of it.

"Shall I________________? Possible responses: Yes I shall. No I shall not. I "shan't" (an older form of English, perhaps appropriate to the 1800's. We can also say, in the more modern form: "I will. "I will not." .