A 46 fin on the IS101 - my favourite and most used board all times - seems a bit too large, even with a 8.5 sail.
The largest sail that I use on my IS101 is a 7.8 with a 39 fin. If you really want to use an 8.5 on that board, I would go for the 42 fin.
What's your weight ?
IS87 / IS101 / IS122 / US147