Using Similar Page Checker (http://www.webconfs.com/similar-page-checker.php), you can check if a web page is similar to other one.
Is there any rule of thumb how high a percentage similarity is accepted by Google? That is, when Google consider that a page is too similar to another one and will not index it.
I have two pages on the same domain where "how to order" information is 70% similar.
Well there would be an end to content duplication and copyrights scandals if this becomes known, the very reaosn why Google or any other search engine would never ever disclose this information.
Suppose a page surpasses the threshold of the level Search Engine has set for content duplication and it decides to take action, it is not known and definite which page SE would penalize; the one who copied the content or the one whose content was copied. Ofcourse this could be traced back as to when the first of those pages appeared on the web, but still it is not definite. So it is better to keep checking for copied content from your site as well.
As for your situation where two pages of the same site have similar content, it depends upon the situation. If it is only one and the only page, it might not do any harm. It is advisable in most of the cases to remove the duplicate page. If you cannot do that, use the canonical tag to indicate to the search engine which page it should consider while crawling. You can also noindex the page if you can afford to do that, though that would not be best solution. In general, keep the content of each page of your site unique.
Canonical Tag is used like this.
<head>
<link rel="canonical" href="http://www.example.com/" />
</head>
This resource on content duplication might give you an insight if you are a newbie..
I agree with Coding Mash about the canonical, but if you have 2 different ordering pages, i'm assuming that is because it's based on the product, so you would not want to use canonical.
In my experience, very recently i must add, the simple fact of having different meta description tags has been enough to add to pages identical.
Having the same content, but even in different order will be considered different content. Therefore, 70% equal, is not equal at all. You should be fine.
The real answer is to stop worrying about things like this and concentrate on making good web pages.
It's easy to judge yourself whether two similar web pages each add enough unique content to warrant them being separate pages and prevent users from getting confused.
