Need Help Calculating Based on the Value from a Drop-Down List

Hello Again!! 

I have two more form fields that need custom calculations and I don't even know how to begin writing these correctly. Would someone please help me out with the calculations below? 

First Calculation: I have a Drop-Down Field "LAPUsed" that has only two possible choices: Yes or No [Note that in order for me to center these I had to put several spaces in front so the actual values are "           Yes" & "            No" (11 spaces in front of Yes & 12 spaces in front of No) - Not sure if that's relevant or not]

Back to the problem:

a) If the "LAPUsed" Drop-Down value is Yes then I need the text field "TotalofWaivedPayments" to show the Product of "BasePayment" x "#ofPaymentsWaived"

b) If the "LAPUsed" Drop-Down value is No then I need the text field "TotalofWaivedPayments" to input $0.00 (or just 0 since my field is formatted)

Second Calculation: This is similar to the first Calculation, however just a couple more steps and different fields invovled. I have another Drop-Down Field "DispoFeeWaived" that also can only be Yes or No (Same amount of spacing as above so I could center the values)

Problem:

a) First Scenario: I need the text field "EstimatedCharges" to calculate the following:

"TotalofRemainingPayments" minus "TotalofWaivedPayments", then add "AmountofExcessMileageDue", & lastly add the "DispoFee" if the "DispoFeeWaived" Drop-Down value is No.

b) Second Scenario: I need the text field "EstimatedCharges" to calculate the following:

"TotalofRemainingPayments" minus "TotalofWaivedPayments", then add "AmountofExcessMileageDue", & if the "DispoFeeWaived" Drop-Down value is Yes nothing additional needs to be added.

I think that's it for the time being, and again any help would be GREATLY APPRECIATED!!