how to change index cluster ratio?

I am using 8.0.3

One problem I found is that even if I specify to create a clustered index by
"cluster idx_name on table_name",  I still can not tell the optimizer that the
index of "idx_name" is a clustered index with "index correlation" value as 1.0
(traced from the cost_index() in costsize.c)

so where is the "index correlation" information stored and how can i update it?

thanks
Huaxin