Clay, it sounds like you have your plan but if you really want to model your situation start with this spreadsheet.
(broken link removed)
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.
(broken link removed)
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.