Perform a find for all records dated from 60 days ago to today's date.
Enter Find Mode
Set Field [YourTable::yourDateField ; "> " & Get ( CurrentDate ) - 60 ]
Perform Find 
Now a summary field that computes the Total Of your Rainfall field will return the total rainfall for your time period and
500 - YourSummary field in a calculation field will produce the result you've specified.
I think you're talking over my level here. Is the "Perform a Find" procedure a script?
I made this database 7-8 years ago in AppleWorks and am trying to redo it in Filemaker but the commands are not the same. It Appleworks, I used this:
Volume is the amount of rain on a day.
60DaysAgo = NOW()-60 Note - NOW is not dynamic, I have to reset it each day I print the report, unfortunately.
DateTest = If('Date'<'60DaysAgo',0,'Volume')
DateTestSum = 500 - sum('DateTest')
I can't even remember how I came up with this; it's the only database I ever wrote. It seems close to your description but the Find Mode confuses me.
What you've posted appear to be definitions for different calculation field.
I've described a script. Each step matches a step you can create in the script editor for this as long as you subsitute your date field referece for "YourTable::yourDateField".
The second step is to define two calculation fields, a summary field that computes the total "rainfall" over all the records found. The second field subtracts this total from 500.
I put in the script as you described but it doesn't run. All I get is 500 minus the last recorded volume. How do I make the script run? I have searched all over various Filemaker training videos and manuals and all I can find is how to write scripts, never how to make them actually run.
You can select the script from the scripts menu, assign it to a button that you click, assign it to a script trigger or you can highlight the script in Manage | Scripts and click the button with the green arrow.