For this part of the charge rule the problem is that the interfaces do not support the specific type of charge that you would like to apply. Since the interface is too limited for this type of editing you will need to setup the rule as best as you can, then after the fact you will need to go into your WA_eCart folder and the WA_Store_Cart_PHP.php file. In here you will need to identify the code for you charge rule that you crafted for more than 25kg and edit the code to work with your criteria for this charge.
The idea for this customization will be that you need to take the total weight, then subtract 25 from it. You will then need to divide this result by 5, this will give you the number of dollars to charge for any weight over 25. You will have to determine how to round the values to fit your needs after dividing by five.
For example if you had items in the cart totaling 31kg then this charge rule would need to first subtract 25, this would leave you with 6. You would then divide by 5, this would give you 1. So you would add 1 to the base rate to account for the extra 6kg. This is the part that you may want to round, if it is greater than five do you round it up to the next multiple of five, or do you round down.