Pelli Robson chart contrast decreasing factor

[mauroventura]

Hi to everyone.
someone knows the best way to determine the contrast decreasing factor for each triplet letter grooup in Pelli Robson Chart??

Many THanks in advance

Mauro.

[drk]

No idea, but I'd guess that it's 50% less each line down.

Probably starts at 50% and goes down to 12.5%?

Ergo,
50%
25%
12.5%