These are from when we bought the Monterey for our truck. She was 5.5yrs old, I'm not sure what her height was but it might be in one of my old posts from when I first posted pictures of her in the seat. She was in the neighborhood of 45lbs I think but don't quote me on that.
I think I moved the headrest up one position from the one where my hand is on the edge of her shoulder, but that's not something I remember for sure anymore either.
This one is more recent. My dd would've been 7yrs old, about 54lbs or so, and a good bit taller than your dd - she's about 52" tall now and this picture was taken just a couple months ago.
eta: The gray seat is the original Monterey. The red seat is a 2010 date of manufacture. I'll have pictures of the newest Diono Monterey in about 5 weeks or so when I get my seats from them and I'll be able to compare the fit between them all then if anybody wants a comparison. :thumbsup: