InfoPath Dev

I have a list with a "effective date" column and an "amount" column. I want to auto-populate a field in the form with the most current rate. To do this, I'll need to lookup the closest date from the date I have in the form and populate with the amount. How can I perform that filter/lookup?

 Assuming you are using an ordinary list connection, the formula would be something like this (I'm abbreviating here):

SharePointListItem_RW[translate(substring(EffectiveDate, 1, 10), "-", "") >= translate(substring(FormDate, 1, 10), "-", "")]/Amount

 Note that in order for this to work, the connection would have to be sorting the results in order of the effective date (ascending).