Just pay the amount you want to pay (X), and they can calculale...

Just pay the amount you want to pay (X), and they can calculale the number of shares (Y)

Y= X/5.25
Easy

Don't be distracted by the instructions. (need to take up 100% before you can apply for top up). If the amount you send in is lower than 100% of your entitlement, then you get the amount you want, maybe 80%. If the amount you send in is higher, then you get 100% of your entitlement plus whatever amount is over and above.