I am heavily invested in Garmin, from aviation g1000 PFD/MFD instrumentation to my Epixgen2 watch to 1030+ Edge cycling computer. I have a 10 year old digital scale w/ BF metric as well. Today arrived my new Index S2. Read my stripped body weight on the old one: 151.8, BF 18.4%, BMI 24.5%. New S2: 152.3, BMI 24.8% and get this. BF% 41% Back it goes
I had a DEXA scan, then made the adjustment as recommended on Garmin's support site. It's been spot on ever since. If you have no plans of having a DEXA scan, I would just use 18.4% as the adjusted BF%.
Set Body Composition
If you would like to update the reference values for the body composition metrics measured by your Garmin Index S2 Smart Scale, you can do so using the Set Body Composition setting. We recommend only updating these values using recent measurements from a trusted source, such as a DXA scan or another reputable body composition measurement.Once you enter new values for these body composition metrics and save your changes, these values will be used as reference points for estimating new metrics each time you weigh yourself with your Garmin Index S2 Smart Scale.
Also, verify that you have the latest firmware release. Currently 3.30 - Manually Updating Software on the Index S2 Smart Scale | Garmin Customer Support
