The PAYGI Instalment income estimate (xPI) is used to calculate the Commissioner's rate based on notional tax payable. This is the rate the Commissioner uses in your PAYGI assessment.
You may choose to base the calculation on the current income year or one of the two immediate prior years returns. The ATO will have based the Commissioner's rate on the last tax return lodged, not the one you lodge today.
It is not advisable to use both the xPI and the xPG for the same client. If you make a mistake, it is easy to redress.
Delete the worksheets and select the one most used by your practice. Re-creation is immediate upon selecting the year to base the calculation on.
If you subsequently change a value in the income tax return that the PAYGI calculation was based on, you must delete the schedule and recreate it to see the changes reflected in the calculation.