The tax rate is applied to the Production Tax Value, PTV, not the price per barrel.
The PTV is simply the Price – Transportation Costs – Lease Costs per barrel for legacy oil. Plugging in numbers from the latest Alaska Revenue Source Book, the PTV equals $50 - $10 - $28 = $12. 

The PTV calculation for NEW OIL also includes an additional deduction of 20% of the wellhead value; ($50 - $10) * .2 = $8. Therefore the PTV for NEW OIL is the $12 - $8 = $4 per barrel.

So legacy oil, BEFORE production tax credits are applied, is taxed at $4.20/ bbl. and new oil at $1.40/ bbl.

Now the magic really starts; applying production tax credits, the minimum tax floor and offsets, carry forward of operating losses, annual true up of unused monthly production credits, etc.