Re: Missed Target O.G. for my Sweet Mead....
I would let your Mead ferment through to completion, rack off the lees, cold stabilize, rack again, take the SG, then leave it for a few months to age. Test the SG again to ensure it has stopped fermenting, then when you are ready to bottle, backsweeten to taste using the same honey. Be careful not to aerate the Mead when mixing in the new honey.
To know how much honey to add, take a measured amount of Mead, and add measured amounts of honey until you reach the sweetness you are looking for. Then, you will know the ratio of honey to Mead to add to the 5 gallons.
This is, of course, if the end product before sweetening is not what you want. With your OG readings being so close to the one you were aiming for, you may find it is just right.
Chan fhíach cuírm gun a còmhradh
A feast is no use without good talk.