The cursor offer a Copy Range command with which you can create interactive ranges from data sets.
1) Use the command Copy Range on the Cursors toolbar or in the Cursors menu.
2) In the dialog box that appears, specify whether the current cursor positions are to be inserted as fixed or variable values. The command saves a formula in the clipboard, which extracts the section from the curve. In the latter case, the properties of the diagram, worksheet or document are used that determine the positions of the cursors. Thus, for example, you can combine the manual selection of a signal section with an automated analysis.

Now you can create an additional FPScript formula to calculate the response time. Use the name of the FPscript formula (with the data range) as input source.

I’m not sure what’s the total signal amplitude. But there is the Sum-function to calculate the sum of a signal.
Example:

0.63 * Sum(SignalSection)

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