The 1% annual fee is multiplied by the account value at the end of every day and divided over the total trading days of the year (252 days). Then, the fee at the end every day is multiplied by the trading days of a given month (21 days in a month) and charged on a monthly basis in arrears.
1% X $20,000/252=$0.79 (fee for that day)
[Average 21 Trading Days per month]
$0.79 X 21 (Trading Days) = $16.67