sure is by my guess! I don't have any tools for testing waterproofness yet!

anyways common sense will suggest me:

if you don't wave your lens during a rain but you keep it in your bag

it will be waterproof with a "light rain"(intended like 1mm of rain per hour, or 0,2641 gallon per square meter of rain par hour) all the way to a medium rain (4mm of rain per hour , equivalent to 1,0564 gallon par square meter of rain per hour). on hard rain (7/8 millimeter per hour I will may be not keep it in the rain for too long , but I m pretty sure that if you did well the lens bag it will be all right ('cause plastic and innertube are waterproof, and this is not my guess).

like I wrote in my instructable it depend on how the shape of your bottle and how tight you can make it.

