Provided always, that where such parish shall not be altogether situated in the same county or where no fiars applicable to the kind or description of grain modified shall be struck in the county wherein such parish is situated, it shall be competent for the said lords of council and session as commissioners aforesaid, to convert the said money into grain or victual according to the average of the aforesaid seven years of the fiar prices of two or more of the adjoining counties, or of such county or counties as they shall deem most suitable in the circumstances of the case.