My thoughts on this are the following: (I hope you can get under the deck or else nothing much will work)
Where the last deckboard is, screw a 2x2 or 2x4(on end) under the last deck board, flush with the front face/edge.
Crawl under the deck and from behind, fasten deck board (5/4" x6") flush with the top of the last deck board - essentially you will drive screws through the 2x2 or 2x4 you just finished mounting into the backside of the decking you're putting on as a fascia board - use appropriately sized screws so tehy do not come through the wood.
The end result should look pretty good and leave you a 1/4" gap which is perfect.
I have placed a cross sectional diagram below:
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