Hi, can someone help me calculating an standard deviation? it's from a vitamin D bloodwork that I got done because I suspected it was low as I work with computers and spend all day indoors and only go out at night.

Suppose result for Vitamin D was 20.2, range is from 12-60 (lower than 12 is severe deficiency, higher than 60 is an overdose). So yeah I need to get some sun. since it's on the lower end.

Anyway, they gave me an "SD%" value, this I think means "standard deviation". This value is of -32.9%

My question, how is that calculated? I mean how do you calculate the standard deviation of the result given that range?

Im not a numbers guy so I would like to know how that works.

Pic somewhat related.