Yes, notoriously difficult, which makes your Breitbart number even more questionable. You still don't even know what that number represents. "Per month" can indicate it's averaged over all months, for example. This would cover up its recent downslide, after its huge upsurge in 2016. The Breitbart source itself even cites Alexa, which is referenced in my source as a corroboration to the numbers it cites.