In 13.10 Database Design documentation it is the section 'NUSI Sizing Equation' and 'NUSI Space Considerations' in 'Chapter 15: Database-Level Capacity Planning Considerations Sizing a Nonunique Secondary Index Subtable'
I usually use a bruce force approach. Not very sophisticated...
1. Create a 1:1 copy of the table.
2. Insert select some data with sample like insert into copy_sample select * from source sample 10000000. The sample size depend a bit on your system size. In small configurations might be 1 mio sufficient.
3. Measure copy_table table space
4. drop nusi
5. Measure copy_table table space again
6. calculate NSUI space as % of diff
7. calculate source table space and weight with the % of step 6 -> approx. NUSI space.