Snapshot for a look-up field?

Hello,

I have a table that contains my sales reps and their commission plans.

Whenever I generate a project in my projects table, it links the sales reps' commission percentage to each project.

I have a 3rd table (Costs) that calculates fixed costs on projects and calculates commissions related to this information. The look-up numeric field pulls the commission amount from the projects table.

I'm trying to make the look-up field in Costs for the commission percentage be a snapshot, as contracts and commissions change, i can't have it affect older projects that were created during a different contract value.

Whenever I change the field to a look-up field, the snapshot option disappears.

Can someone help me with this?

Thank You So Much