While the answer from @Jerry shows a solution that @James Deadman might need, mention should be made of how Excel will calculate the percentage of conditional formatting using icon sets.
Assuming the following sheet:

In A1:A6 is the same conditional formatting as in the question.
So how does Excel calculate percentages? It takes a minimum of A1:A6 as 0%, and a maximum of A1:A6 - 100%. Thus, the percentages in column B are calculated:
=(A1-MIN($A$1:$A$6))/(MAX($A$1:$A$6)-MIN($A$1:$A$6))
how the formula in B1 copied down.
To understand how this relates to the question:
In the example shown with the image, it seems that conditionally formatted values ββare percentages. Thus, the maximum is 100% = 1, but the minimum value is the lowest percentage value, possibly 90% = 0.9. Thus, 1 = 100%, but 0.90 is taken as 0%, since this is the minimum value.
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