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Well you can, theoretically, but it can get very technical or be a bit imprecise. You are correct that you can't really guarantee a set number of leads (what if everybody's plumbing is working perfectly in London this month) but you should be able to track your contributions. If you use a second phone number on Google Local and the meta description, then anybody coming through that is a SEO lead. Also, while a bit wishy washy, you should be able to see in your analytics what percentage of people are coming through on unbranded search and then attribute a percentage of web completions to that. You get more precise by setting up a tracking system, but it'd be a very bespoke set up.