Good luck with that as most/all smart scales are inaccurate.
Is BMI an accurate way to measure your health? Yes and no. Learn how to measure your BMI, why it’s still a useful tool, its limitations and when to talk to your doctor.
health.clevelandclinic.org
I guess can use one's number as a "relative baseline" and track relative to that (ie. see if going up or down), but to say "my BMI is X", a bit wishful imo. Or get a DEXA test, see what scale says that day, see how far off and adjust accordingly from then on to have a relative idea what the real number is.