No matter what the stated deduction from WTI is supposed to be to get the wellhead price the last 2 quarters with low prices had just over $7.00 deducted.
Average WTI price of $57.80, taxes were $2.02 (4% of 50.50, 7.30 deduction)
Average WTI price of $48.80, taxes were $1.67 (4% of 41.75, 7.05 deduction)
Within a quarter negative royalty days are subtracted from the positive royalty days.
3rd quarter 1998 10-Q
"Furthermore,given that the calculation of the total Royalty Production for any given quarter
is comprised of the sum of the product of the Royalty Production for each day in
such quarter, positive days of Royalty Production (after deducting for the
Chargeable Costs, Cost Adjustment Factor and Production Taxes) will be offset by
negative days of Royalty Production (as long as the sum for the entire quarter
does not equal less than zero)."