Calculate stepped fermentation gravity

I had this normal Oktoberfest lager (SG 1.058) going, and decided it was boring. Sooooo I decided to do an experimental double decoction and add it to the fermenter a week later. I am trying to calculate the new starting gravity of after I added the decocted wort. Can I just use a weighted average formula to calculate that? The original wort was 1.026 (6 gallons) when I added 1.5 gallons of 1.20 wort. Does that mean the new SG is 1.061? Thanks!