to 100% guarantee that you are infact under the legal requirement that you mention, you are probably going to have to get it tested by an independent testing authority of some sort... short of that, do your best, to me it sounds like you are sitting ever so slightly under the legal limit.

Myself, I have been well over the limit in the past(26db router and 24db antenna), with no-one in a black helicopter telling me to shut it off, however I now go a bit under the legal requirements.