Why speculate? _Anthropometric Reference Data for Children and Adults_, Table 20 "Waist circumference in centimeters for adult males aged 20 and over and number of examined persons, mean, standard error of the mean, and selected percentiles, by race and Hispanic origin and age: United States, 2015–2018" is right at https://www.cdc.gov/nchs/data/series/sr_03/sr03-046-508.pdf
The mean in the very first row is 102.9 cm (the 40.5 inches in the OP), while the 50th percentile is actually lower at 101.8 (39.7 inches).