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IMO there is no fixed % or ratio when it comes to the use of synonyms. The idea is to have as much relevant content as possible whilst not coming out as being to spammy. It also depends on how much content you have on the page.

we recently started using Senuke and the results are not too bad given the cost. With regards to it being a long term strategy, NO, id suggest you use it for sites that you do not plan on keeping for a long time or sites that you want to make quick money with.