When I switched to a new lot of Varget I had to drop the charge exactly 0.9g to achieve the same velocity with the original lot. Both lots were fired the same day through the same chronograph. Here's a thread I made about it.
http://www.snipershide.com/shooting/snipers-hide-reloading/203254-new-lot-varget.html
http://www.snipershide.com/shooting/snipers-hide-reloading/203254-new-lot-varget.html