The Friel Charts above are a good guideline. General rule of thumb for 70.3 is 0.7-0.8 IF, the faster you are on the bike and the more confident you are in your run the higher you can go above 0.8.
It also depends on your ability to pace and to adjust your effort mid-race. A 0.8 IF evenly paced is much more forgiving than having to deal with a VI of 1.2 or greater. Also, some days you feel strong, others you don’t, pushing the upper limit of IF requires the ability to adjust mid race, to know when you can push and when it might be better to back off.
I’ve began riding around 0.74 when I first took up 70.3 racing a few years ago, this year I rode 0.82 and 0.83 followed up with two sub90 runs, so definitely not overbiked.