estimate btree index size without creating

Dear all,

Is it possible to estimate btree index size on text field before
creating it. For example i have a table like this:

fortest=# \d index_estimate
i          integer
somestring text

This is avg bytes in text field
fortest=# select round(avg(octet_length(somestring))) from index_estimate ;
4

And number of tuples is
fortest=# select reltuples from pg_class where relname = 'index_estimate';
10001

Can i estimate index size, operate only this data, may be exist
formula or something to do this estimation.

--
Best Regards,
Seliavka Evgenii