Refactor HRPD prompt pulse subtraction for readability

[RichardWaiteSTFC]

Description of work

Following on from PR #37929 - this is a small refactor to make the main method a bit smaller.

Fixes #37969

To test:

(1) CI passes
(2) Follow instructions in previous PR #37929

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[MohamedAlmaki]

LGTM, I have tried the script and it is working as expected. I have small comments about confusing expressions. Thanks for refactoring the code, which improved the readability of the code

[MohamedAlmaki]

LGTM, thanks for the refactoring which improved the code readability!