Whatever size prospect list you have, make it smaller. It doesn't matter if your list is 10 names or a database of thousands. You have to start somewhere. And there is no better place than at the top.
The particular score is less important than having a consistent methodology to compare prospects relative to each other. Create a score that makes sense for your constituency. You may want to add other variables, such as membership, ticket holders, event attendance, volunteer status, etc.
However, keep it simple. Do include hand review of your top scoring donors.
Detailed data on your donors' giving behavior is unique to your organization. Use it to your competive advantage.