1) I gather that this is a Running Total Summary--which will work in reverse of what you want due to the reverse order in your portal. To get the correct order, add a second summary field with the same settings except do not specify that it be a running total. Then use a calculation field with this expression to get a "reversed" running total in your portal:
NewSummaryField - RunningTotalSummaryFIeld
The new summary field will compute a "grand total" for the records in your portal and this calculation will then get a descending running total by subtracting from that grand total.
2) This sounds like something better produced from a summary report using sub summary layout parts instead of a portal: Creating Filemaker Pro summary reports--Tutorial
The problem with: NewSummaryField - RunningTotalSummaryFIeld
is that the reverse running summary that you achieve only gives the result with the next record, ie. if you'd added 3 seconds of activity, the cumulative total would only show this on the next record.
Could I not instead use: Time: categorX+GetNthRecord (CategorXSummary; Get ( RecordNumber ) - 1 ). The catch is, how do I start this summary because for the 1st record there won't be a Get ( RecordNumber ) - 1?
Let ( R = Get ( RecordNumber ) ;
If ( R > 1 ; GetNthRecord (CategorXSummary; Get ( RecordNumber ) - 1 )
But I do not recommend using GetNthRecord in this fashion for any but very small numbers of records. You can easily max out system resources and get ? results.
You may be able to use a method that uses getNthRecord as an auto-enter calculation to copy a value from the preceding record and this avoids this issue, but can create problems in getting these totals to update correction in all situations.