The cost basis for the sales to cover expenses can be determined from this document.
If you take the number from the third column and divide it by the second column, that will give you the fraction of silver used to cover expenses. Multiply that by your total cost basis to get your cost basis for the sale.
Then for the next month, you reduce your cost basis for the sale from the previous month. @Jack100000 has suggested an excel spreadsheet to calculate this for you in the bottom of this post related to GLD shares which has similar reporting.
