HFI Calculator: PGFC precipitation sensitivity

**Actual Behaviour**

" Brett was also reporting that the HFI values in the PGFC appear to be very sensitive to even light amounts of rain.
Can you verify the calculations are correct? "

**Steps To Reproduce**

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**Screenshots**

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**Expected Behaviour**

A clear and concise description of what you expected to happen.

Start by locating the HFI Calculator and PGFC precipitation calculation entry points in the Python repository. Reproduce the reported sensitivity to light rain, compare the resulting values with the expected calculation, and document or test the verified behavior so completion is clear.

Written by the indexing model from the issue text.