P means phosphorus, Sb antimonium, Sn tin and As arsenic. I don't know of a steel containing antimonium, tin and arsenic. As for phosphorus, its percentage in steel is always kept to traces, its maximum being specified by standards like SAE, ASTM, AISI etc.
Sorry, I can't help you, I don't realize what the purpose of Bruscato formula is.
Giovanni S. Crisi
Sao Paulo - Brazil
PS. There's the chance that Bruscato formula is not intended for steel, but for other metals, e.g. aluminum.
Gentlemen, you (me, in this case) learn something new every day, and I thank you for having taught it to me.
I didn't know that just minimal quantities of phosphorus, antimonium, tin and arsenic could have such great influence in welding chrome moly steel. Back in my days of erector engineer (many many years ago) I welded tons of chrome moly steel (well, not me but the welders under my supervision) and the mean concern was preheating, PWHT and cleanliness. The x factor didn't even exist.
I may add something to the excellent article on The Fabricator magazine: ppm means parts per million, i.e., milligrams per kilogram or grams per metric ton.
Giovanni S. Crisi