I don't think you provided enough information or I don’t fully understand what you are asking. VMG with the FI501 shows VMG to windward. It uses either STW or HDG for the calculation. In your case, the STW is your DST800, but what is the source of Wind? If you have a Furuno Wind sensor directly connected (FI5001) then any Offset /Correction applied should be taken into account in the calculation. If wind is coming from some other source then any wind correction would have to be applied at that source device. Have you tried setting a large offset and seeing how it affects your VMG? Below is an attached drawing you might find helpful.