BMI is a commonly used measure to determine whether or not a person is at a healthy weight. A person is said to be overweight if they are above 25 and underweight if below 18.5. This question asks what the scientific basis is for BMI ranges, but does not question the difference between the values within that range.
Between the upper and lower "healthy" limits is a rather large range of weights. Anecdotally, when I am at a BMI of 25 I still look and feel rather overweight (flabby, bulging belly, etc.). I've only ever been as low as around 21 or 22 BMI, at which point I am rather slim, and at the weight I'd need to be to achieve 18.5 I imagine I would be very skinny, too slender by my personal standards.
Is it really the case that someone at a BMI of 18.5 would be equally as healthy as someone at 21.75 (the midpoint) or 25? By healthy I mean equivalent risk of long term illnesses, early death, etc. (assuming other health-related factors to be the same).
If not, then is there a more accurate way to determine an optimally healthy weight (as opposed to merely a range of healthy weights)? Having been unable to find any information on this point, my target weight is a BMI of 21.75, but I would like to fine tune this if possible.