I think it is difficult to determine the exact number, but you can calculate an average. To do this, you will need to track your traffic and the number of customers per month and calculate the percentage. If the percentage is less than 50% and you buy advertising, etc. then it is worth paying attention to whether it is located relevantly and after that draw another conclusion.
It's not really difficult. On example out of 1000 monthly traffic you get 10 new customers that's 1%. So only 1% from the total traffic actually converts.