Anyone know F nat?
There is no (single) answer to this question!
That is without making an assumption:
A notes pitch/frequency when using "just intonation" depends which degree of the scale it is.
Let us assume that since you are saying F# is -14cents that you are considering a D scale -
One possible approach would be to take the 'minor third ratio' 6:5 (from the relation of the third to the fifth degrees of the scale) this would give you an F of about +16cents compared to ET if your D is 0cents to ET or +14cents if you're tuning the A (and your D is -2cents)
http://en.wikipedia.org/wiki/Just_intonation
From wikipedia:
In music, just intonation (sometimes abbreviated as JI) or pure intonation is any musical tuning in which the frequencies of notes are related by ratios of small whole numbers. Any interval tuned in this way is called a pure or just interval. The two notes in any just interval are members of the same harmonic series.
So if your F is being used against a Bb then it would use a 3:2 ratio which is 2cents wider than a 'ET fifth' - the actual pitch of the F would depend on the pitch of the Bb
Well that's probably more verbiage than you were seeking, and yet probably not enough!
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