Potency of Mexican Brown Powder / Black Tar

[soma]

SWIM usually gets yellow or south east asian (china) white. However, SWIM was out buying some blow and morphine in the projects (lol) and the guy had some Mexican brown powder, so while SWIM was there he got some.

He knows the Mexican brown powder is black tar heroin cut down with a powder to aid in snorting. However, he doesn't know what the potency of black tar is. He watched the guy cut it down from tar to powder, so he knows about how much tar is in it. What SWIM doesn't know is what the potency of tar is like... He hasn't done much smack lately, but has a ~240mg/day OxyContin (oxycodone) habit. He would estimate that each 'dime' of powder is roughly 1/8th of a gm of tar, so with that in mind, where should SWIM start? How does this compare to yellow or white? (and yes, SWIM knows its poorer quality, just how much he doesn't know)