The custodian's calculation is correct. I have no idea how you are coming up with $7,500. If you are saying that the value of the shares of Stock A purchased with the $7,000 has now risen to $7,500, that is not how you are to determine the attributable gain.
"(but the earnings due only to the Stock A purchases are much less)."
This statement is irrelevant. The calculation is is required to be done on the value of the entire account, not on the performance of any particular investment in the account..
