Naphtha should generally be equivalent to heptane. When we commonly talk about naphtha here, take in account that naphtha is meant as a mixture of aliphatic hydrocarbons, (pentane, hexane, heptane, octane, etc). In this case you can consider naphtha equivalent so measures are around the same.
At times, it can happen that a product sold as naphtha (this is the case in parts of europe for example) but it has some aromatic hydrocarbons (such as xylene) also mixed in, which would either change the numbers or even make it unusable for recrystalization (because impurities can also be very soluble in xylene). So make sure your naphtha is aliphatic hydrocarbons, and then yes keep around same ratio for recrystallization.
Regarding the undissolved goo, yeah its possibly active oxides (though Ive never heard of anybody actually chemically analyzing that goo to know for sure, so I guess its only educated guesses at most at this point), plus other impurities. Actually I've noticed there might be even some normal dmt still there, so I recommend doing a second small wash (like lets say another 10ml warm naphtha), and freezing that separately, as it will likely have some dmt but also a bit more impurities than the first wash.
As for acetone re-x, what do you mean? How could you re-x dmt with acetone? Acetone dissolves too easily all impurities and it wont freeze precipitate... Acetone re-x can work with dmt fumarate, but thats a different story