adding a constant after a certain weight

Hi,
I have set weight rules as well, but from 50 kilos I need to calculate a minimum of 26$ + 0,45 cents for each kg more
so if 51 kg = 26,45
52 = 26, 90
and so on
is there a rule that I’m missing here?
Can I integrate with a php code in the plugin?
Thanks for the reply