I've edited my Apple Numbers Weight & Balance spreadsheet to allow overweight flights with ferry tanks and some flight planning tools. It's designed for a 1980 A36 with Osborne 20 gallon tips but can easily be amended for other configurations.
The grey cells are for defaults - set once and forget. The yellow cells are for values that change from flight to flight. The blue cells are calculated.
Please check it out and let me know if I've made any mistakes!