I just want to say that you can't really practice Tummo merely by reading a book.
I mean no disrespect, and don't mean to antagonize anyone.
I've read Mullin's translation of the Tsong Khapa Six Yogas commentary, as well as Mullin's other writings on Naropa's six yogas. I own them, actually.
In my Kagyu lineage, these practices aren't publically discussed, frankly. They are advanced practices, done in retreat, and only after a considerable amount of "preliminary" practice of various sorts. Even talking of a "session" of Tummo practice, Guru Yoga is essential. I'm pretty sure Mullin writes this, as well. And I also want to point out that these practices are completion stage practices of Highest Yoga Tantra, so one would need at least one Higher Yoga Tantra empowerment from a qualified teacher in order to really practice Six Yogas.
That's not to say whatever you're doing is "bad." It may, in fact, be good and useful. But it's not truly "Tummo" practice.
I thought it was hazardous and useless (at least in gaining realizations) to do/attempt Tummo (it's visualizations) without a guru. What would be the benefits? What are the risks of doing the visualizations without a guru?