The purity should be written in the product description on the website you bought it from. If not, it should be written on the bottle's label. Note that, to help stop the banning of SN, it is advisable to remove the label on the bottle, so when it is found, nobody knows what was inside.
Look up " safer injection guide psychonaut" in google.
Lostallhope website says the MLD - minimum lethal dose of Methadone Hydrochloride (brand name Dolophin) is 19 pills x 5mg. Now, since your methadone is probably mixed, it is unknown how much you would need.
DXM, MXE, Cocaine, GHB, GBL, GHB, benzodiazepines, alcohol, tramadol and ketamine all make methadone stronger when taken with it.
However, I would advise you against using Methadone, because we have no reliable info on how to ctb using it. We don't know the correct dosage - 350mg could not be enough to die. I would say, try SN. We have a megathread here, with some 250 pages of opinions, experiences etc.