First, the excess above the current rent is NOT a down payment. It's a rent credit, or option fee.
I'm not a lawyer, so this isn't legal advice. However, you really want to have the excess considered an option fee so that you can retain it if the tenant-buyer doesn't purchase. Also, calling it a down payment suggests a sale is occurring--which it may not. Thus, if the tenant ends up not paying rent, you might be forced into foreclosing, rather than evicting. You don't want to do that.
But to address your specific question: Whatever you want. Frequently/typically, it's several hundred dollars (depending on the amount of the fair market rent). And, looked at another way, often about 25% of the option fee is credited toward the purchase price. But it's really all negotiable.
Example: Fair market rent is $1,000. You might charge $1,200 a month, with $300 per month credited toward the purchase price. Example: Fair market rent is $1,600. You might charge $1,850, with $400 a month credited toward the purchase price.
Again, really, all that is negotiable. You might decide to charge only fair market rent. Or you might charge more, but give a really hefty credit.
Hope that helps.