ok.

First, we multiply AC voltage by 0.9.
then, we multiply the result by 1.41.

There, you have the result for DC voltage.

In our case here,
we muliply 220VAC by 0.9 = 198
then we multiply 198 by 1.41 = 279VDC.

This calculation is for full bridge rectifier with an elec-cap.