Update: Live and learn. Bullet alloy blends are identified by the % of weight of each element in the alloy.
Recently I read an old article by Jamison, in "the Art of Bullet Casting" published by Wolfe circa 1981. I thought I understood his methodology, and in order to answer a question I had about bullet alloy I contacted Rotometals, and the % of each element in a bullet alloy is by weight.
I read the Rick Jamison article in my hardcover "the Art of Bullet Casting". This article was first published in the July-August 1974 Handloader Magazine #50. I was unable to find it online.