Yeah you gotta be careful with BMI calculator
They can be a pretty iffy representation of your health, particularly if you're on the upper end of BMI, as it doesn't take into consideration if you're a muscly person, or are small, yet muscly.
You can obviously be a larger person in that way, but be in good health.
But there's not quite as much to validate it if you're hanging at the lower end, unless you're very tall.
At the moment, Donnie sits firmly 1/2-3/4 up his ideal BMI. Which is a good place to be.