How Can I Charge For SEO Services, Based On Sales The Client Makes ?

I was thinking to do some seo work for a website, than based on the sales the website has, to charge the client.
So if for example my client is a masseur, he would get clients through his website thanks to my seo services. Then i would like the client to give me a percentage of his money only based on the clients he has had coming from the website.

I'm wondering the next things :
1. how much percentage to ask, based on the sales he makes
2. how to keep track of the clients he gets from website (what kind of app can i make [or find online] that would help make appointments for the services the client offers, but also be able to make appointments based on the available spots the client has)

You are talking about PPL marketing.
You should charge in a per call deal. Usually between $20 to $50 each.
There are phone call tracking services that record this calls.
Rent one of this phone numbers, display it on the site or a landing page.

It is not a good idea to do that.

If you are selling the leads on a 100% commission base, you should own the site, then if they do not pay you can always sell the leads to their competitor. If you rank their site and they dont pay all your hard work is lost and you get nothing.

So, if you rank their site you should always charge a fee or go for a hybrid, part fee, part %. That way you will guarantee getting something for your hard work.

Charge a monthly fee regardless of whether your client gets more leads
Best value for your time.