Hi, BAD.
Don't Edit the gains. Delete the Sell transaction and re-enter it. Rather than enter any cost number, tell Quicken you want to Specify Lots, then use the mouse to point to the correct lot and let Q supply the cost number from its internal data, avoiding any problems with rounding.
Perhaps the rules have changed since I retired (in the 1990's). In order to select lots, we had to instruct the broker, before the sale, to sell from a particular lot. Also, we could not use average cost; we were limited to LIFO, FIFO or specific identification and, once we chose one, we had to stick with it, at least for that security. Have you discussed these rules with your own CPA or with the broker?
I understand that average cost IS allowed now for mutual funds, but I've not actually investigated that.
RC