As mentioned earlier by another, your hand size does matter. I use the pelvic spread more as a "change" indicator. I check all the pullets at about 15-17 weeks. I note each ones spread number. Then as they get closer to lay i begin checking regulary. If done properly, the pullet wont be phased at all.
I had a couple girls measure an easy 2 at 17 weeks, and at almost 26 weeks they still havent laid yet. Also, i had a pullet go from a spread of 1 to 4 after her first lay, because her first egg was a HUGE double yolker.
So when i find a new egg, i do a quick spread check on the non layers and identify immediately who has graduated.
Happy egg hunting