Look about 1/2 way down this page. http://www.centraltools.com/specialty.html
There's some straightedges, then try putting various feeler gages under the center & see how far down is the 'low spot'.
You'll get different opinions on what is acceptable & part of it depends on over what length it is, what compession, & what head gasket you use but I'd say over 5 thou. is asking for trouble just to start the discussion off here.
Hope this helps - Doug

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