As requested:
The terms and conditions for your employer's enhanced scheme are completely up to them. If your pay normally varies week by week, in other words you have no regular salary, they would take some kind of average to work out how much to pay you. Whether they use the same weeks as are used for SMP is entirely up to them.
If the fact that you were paid extra in May is completely a coincidence, more of a one-off, and you usually receive a regular amount, then that will be what they mean by 'full pay'. You will get paid whatever you usually get paid.
They can't pay you less than you would get for SMP, obviously, so if by chance and because you received an extra payment during the relevant weeks your SMP actually works out as more than your usual full pay, you would get SMP for the first 6 weeks, then 'full pay' for the remaining 26 weeks as per your company's policy, then back on to SMP only after that.
If you want a calculation before the event, you would be better asking your payroll department rather than HR, who don't normally sit there doing pay calculations. They will inform whoever does your payroll that you are going on maternity leave and from what date, and will confirm that you are entitled to the company scheme, then the person in payroll will work out how much to pay you.