first of all the 20% reduction does not apply to all balls, bigger balls need a 25% reduction, smaller 15%, at least according to the IPC document I am familiar with

second, understand where this 20% comes from,  how do they determine how big of a ball to put on the BGA? they make it 20% (nominal) more than the exposed pad size on the BGA, that's where the 20% number comes from. Ideally the size of the pad on the pcb should match the pad on the BGA. Some vendors give this info, some not. If they are not increasing the ball size 20% then reducing the pad on the PCB 20% is not what you should be doing. Use the size of the pad on the BGA if it is available.

third, the pad size has to be adjusted to escape the land pattern, this usually means the typical dogbone and via, or via in pad, the size of all this has to be selected so it can be built so it functions as intended, is reliable, doesn't cost a fortune, and as many as fabricators as possible be able to build it. So you may have to change the ball pad slightly from the calculated value.

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