BMI is plagued with issues like being completely stupid and not giving a very accurate portrayal of health. Whereas actuary tables give a pretty good metric of how healthy you probably are.
Rather than apply some retarded formula that doesn't actually have much in the way of useful information. What is my chance of death at this weight vs. my chance at death 30 pounds lighter? Nobody could argue the information is worthless, nobody could argue that the statistics are useless, and nobody could have little nuanced claims like BMI doesn't reflect the massive changes in health for losing even a little weight. Poof.
Admittedly you can't give them in units like 1.3% chance of death vs 1.2% chance of death here. You'd need to invent a different standard of give them a number which isn't their chance of death but looks a bit better.
((Chance of Death) - (Optimal Chance of Death at that height/age)) * 1000
So if you're at 0 or around 0 your fatness isn't going to make you die faster. If you want a better reading feed in more data. We have statistics for chance of death considering muscle mass et al.