If you have a webpage that's on page2 of Google for keyword 'x' and you want to get it to #1 page 1, then you need to be able to beat that #1 positioned webpage, you'll need more quality links for example, you'll need more 'likes', social links etc. You want know exactly how many backlinks that #1 webpage has and where they are coming from in order to beat them (although there is software available to help - eg
seo spyglass ). Therefore you can only guesstimate, plus if that #1 webpage is still getting hundreds of new backlinks per week (plus other pointers) then you'll be playing catchup for a long time.
Then don't guarantee #1. Guarantee top 5, or top 10... or whatever you feel you can deliver.
If the prospect doesn't like that, he can walk.
It's not for him to decide what you guarantee. That's your decision.
Seriously, think about it, what's the alternative? Is the alternative that the client wants top spot, and only top spot, and you tell him you think you can get it... even though you have no idea whether you can?
And one of the reasons that people should try and do their own SEO - in my opinion anyway, or perhaps offer a % per sale
A % of increased organic sales is a guarantee of sorts. If you don't produce results, you don't get paid.
However, there are downsides to that - and, to a lesser extent the same downsides to making guarantees for PPC.
For example, most businesses are seasonal. If you start work as the business is becoming quieter, how do you calculate the net impact of your work?
Or, if a competitor comes along and offers the same products for less, the conversion rate goes down. You lose out because of something that has zero to do with your work.
Ideally you want to guarantee your work, and eliminate as many outside factors as possible.
(Which is why I guarantee my split-testing service, but not my PPC services - the former eliminates outside factors, the latter can't.)
Steve