Hello,
Put this formula <field1 / field2> in the default value of the
totaldepositAmount where field1 is downpaymentAmount and field2 is
totalprojectAmount.
This formula should be kept in the properties of the totaldepositAmount field.
I hope it helps you.

Signature
Kalyan Reddy
Visit http://www.autonomysystems.com
> How can I take the data from a drop down called downpaymentAmount which
> has a percentage value, and divide that by the field totalprojectAmount
> to get the totaldepositAmount field to populate when each deposit
> amount is selected?