Hi! I will be doing the Levi’s GranFondo this Saturday too and I also used Best Bike Split to estimate my finishing time. I am fairly new to cycling metrics but I would suggest looking at past long rides of similar distance and seeing what you were able to achieve as far as IF is concerned. I read that using 0.75 for your IF is a good place to start.
For my last GranFondo (Whistler 2018), I used 0.80 in Best Bike Split but TrainerRoad reported an IF of 0.97 while Strava reported a 90% Intensity number. This was my A-race so I pushed myself hard (and came in 20 minutes faster then the estimated finishing time).
Also, good nutrition and hydration will help in keeping the intensity up on long rides!