Methadone blood level in Toxicology report?

My ex husband whom i was very close to for 30 years died in april. His autopsy report came back and said he had a blood consentration level of 1.5mg/L, and 13mg in his stomache. I dont understand how much that is when it comes to methadone. He also had a theraputic amount of celexa and wellbutin. How much is too much. He was not in a methadone clinic or had a rx for it. We are trying to understand what may have happened the day he died. I need to understand those measurments, He was healthy and was 6ft 2 222 pounds. Can anyone help?