Typically when I start with a niche with some basic keywords (ie: lesbian, lesbo, girl on girl, etc. etc...) I end up keeping only 3-4% of all keywords after I run it through my harvesters. So if you have 590k, I'd venture a guess of 17700 - 23600. That's a rough estimate using my research patterns. It really depends where you pulled the 590k from
WG