Hi @Michbill, I emailed Christopher from
Modernist Pantry to pass along your question and here was his response:
"Hi Jason
I don't know the specific gravity of liquid soy lecithin but it is quite viscous and sticky. We used to carry it and stopped for the very reason that the poster mentioned. It is extremely hard to measure. The reason Nathan recommends it in MC at Home is that it is more readily available in stores than the powder is, which is of course not an issue for us. In my experience the powder works just as well as the liquid. What doesn't work well are the granules that are sold in health food stores as "powder" but have been agglomerated and don't dissolve well."
I hope that helps answer your question.
Thanks and happy cooking!