A Dennis Marshall article I'm looking at says that wheelweight typically has 0.17% arsenic. The question is, how much arsenic is in lead shot? The manufacturer's MSDS gives a wide range that is not precise enough for our purposes. However, I've heard that 1% arsenic is typical.
To add to the confusion, arsenic is a natural contaminant in lead so chances are that whatever alloy you are starting with already has trace amounts of arsenic in it.
The 1-20 formula for adding shot sounds quite reasonable and I'd be surprised if it didn't work. The % of arsenic is not that critical for heat treating.
If you are going to be mail ordering your metal, check ebay. Wheelweight seems to sell there for about $1.50 pound, which is a rip off, but probably no worse that any other mail ordered metal. http://www.ebay.com/itm/30Lbs-Used-Lead ... 1c3cc4b3de
It's sad that wheelweight is hard to come by in some parts of the country. I'm so old, I can remember when service stations and tire shops were glad to get rid of it.