Distance Based Fee Settings Select Postcode?

Hi Kim,
I am trying to calculate the distance based fee using UK Post code
My requirement – first five miles shipping is £0. Then for every 5 miles, it should be calculated increment of £5.
The problem is I am unable to understand how google map API is calculated the distance, is it based on post code or on city/town.
My origin Zip/ Origin city – E6 2AA
The destination is – SM2 5EJ.
As per Google it’s 23.9 miles.
expected result = 23.9 – 5 (first 5 miles are free) = 18.9/5 (Every 5 miles it should calculate the shipping) = 3.78 * £5 (£5 for every 5 miles) = 18.9

actual result – 48.72
not sure how it’s calculating,
Also, I have to put a condition that if the cart is £100 – the shipping is free.. I tried adding the free shipping but it’s not calculated.

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