Clay, it sounds like you have your plan but if you really want to model your situation start with this spreadsheet.
www.eia.doe.gov/neic/experts/heatcalc.xls
You can fiddle with fuel prices and delivered efficiency as much as you want. This is exactly what I did, to a fine degree of detail (my wife would argue too fine), when I was considering my stove purchase. It's simple math with this approach. You'll have to estimate efficiencies but reasonable estimates are provided. I lowered my LP efficiency because I know I have very long duct runs through unheated spaces.
www.eia.doe.gov/neic/experts/heatcalc.xls
You can fiddle with fuel prices and delivered efficiency as much as you want. This is exactly what I did, to a fine degree of detail (my wife would argue too fine), when I was considering my stove purchase. It's simple math with this approach. You'll have to estimate efficiencies but reasonable estimates are provided. I lowered my LP efficiency because I know I have very long duct runs through unheated spaces.