Maximum Fee Value

I am offering a 0.75% discount to buyers to try to get them to use Stripe as the preferred Gateway.
Fee Type “percent”
Fee Value “-0.75”
Minimum Value Fee “0”
Maximum Value Fee “20”

I set it this way so that for larger payments there is a cap on the maximum Discount of £20 per transaction.
I’ve just received large payment of over £5000 and the discount given was £43.69 (in reality -43.69 off total price) and so clearly I have set wrongas it should have been £20.00
Can you please explain how I should use this maximum value fee in respect of a discount?
Best Regards
RT