As you called out in your comment, this would not be an issue if you could use the Standard Rollup Summary fields to count the total leads and converted leads. You cannot use the standard rollup summary to count leads on accounts though.
To get around this you have a few options. You could look on the app exchange for an app that does this for you. The advantage here is it doesn't require any custom coding on your part. I would recommend the Rollup Helper if you want to go this route. (I have no personal affiliation or interest in this product, just one that I have used int eh past and works well)
If you want to go the route of coding, there are a few nifty packages out there that I would look into
Anthony Victorio wrote a nice utility for creating your own rollups. Take a look here
Andy Fawcett also wrote a utility for this as well that you can find here
Once you choose any of these methods you can roll up your total leads and your converted leads. then you just need to create a formula field that divides them.