I agree- the Washington Agency Directors has been the calculator I have used almost since its inception. It effectively uses dose dependent straight line slopes that are low slope for 20mg or less dosage but a high slope for more than 20mg, presumably due to the hepatic enzyme inhibition at higher doses of methadone. Consequently, I stopped prescribing anything more than 20mg a day for anyone for many years, and most have been weaned off of methadone.