I wonder if the forward facing position might influence the length? You know, the calculations may be different if you are at a different height and using a different stroke? Sitting on the thwart and really laying into them is different than sitting up on deck and chopping at it, you know? It seems all the forward rowers I've seen use longish oars, but I've only paddled facing forward,, never rowed that way.
I would use a stick or pole or something to figure a good length to start. Pretend to row with it, or better them, while your boat is in the water. Better than guessing and cogitation, I think.