How much to charge-first possible SMMA client

I am about to go on a call with a potential client in the next few days, but am a bit unsure as to how I should price my services. The potential client is a golf eCommerce brand, who I think, are somewhat well established, but definitely not a big company.

The service that I am offering is Facebook adverts. This would be my first client if I were to sign them, so my lack of experience should be taken into consideration as well.

What I was think was this: If there ad spend is like £800-£1500, then I would charge a 400-300 pound one off retainer payment , and then charge 20% of the turnover or profit made from the ads, . Your help would be greatly appreciated!