Just as another point of reference, I tend to agree with Ken. Although my reference point is an older Mike's Lab course slalom that is only about 60cm wide, I was matching a 50cm Tectonics Mirage with an 8.1 race sail with excellent results. The kit had a great low end, but I was also real comfortable in average winds up in the 15-18 mph range. My weight is approximately 75kgs.
Lastly, the product specification for the Futura 133 does reference a potential fin size up to 50cm. Although the large fin size probably was most likely spec'd to match up 9.0-9.5 sail, I'm sure that it should be suitable for an 8.5, especially if lighter winds are the target.