The back is low enough to be cool and comfortable, but the fit prevents it gaping at the front or back so is perfectly modest.
The whole thing was a joy to sew and I am still a real beginner, the only bit I struggled with was the sleeve fitting, I lined up the centre underarm seam and pinned the fabrics towards the neck, as I got there I was a bit confused about the angles.Thankfully I basted it first and soon realised it was wrong.
INCORRECT
After playing with it for an hour in confusion I realised the bias tape on the shoulder part of the sleeve needed to be parallel to the front neck edge, rather than the side of it. I stitched this in place before pinning, it pulls down slightly on the end of the bias tape and fit perfectly.
CORRECT