The ERS limit on a Retirement Account is based on principal only. Thus every time the ERS increases you have more room in your RA for top ups.
If you're trying to maximize interest and monthly retirement income then it's best to top up your RA on the last day (or second to last day) of the calendar month when the ERS increases. The last ERS increase occurred on January 1, 2021, so a top up (via PayNow QR) on January 30 or 31, 2021, would have maximized interest. It's now May 6, 2021, so if you plan to make a $7,500 cash top up it's best to do that on May 30 or 31 so that you get whatever bank interest you can earn on $7,500 until then and start earning CPF interest on that amount from June 1. Alternatively your spouse (if you have one) or other qualified family member may be able to transfer OA dollars into your RA, although a transfer takes longer to process. Now is a good time to request such a transfer.
Unless the CPF Board announces a change, the next ERS increase is already scheduled for January 1, 2022.