Rick wrote:

I use the volume of the sand. I reason, rightly or wongly, that the
sement sits in the gaps in the sand to "glue" it together.

Most on-site mixing of concretes and mortars is done by volume. The
readymix (supplied to site in mixer lorry) tend to use weighbins as they
batch from their silos and a water meter to achieve better quality
control. If a bad load goes into a big project, it can turn very expensive!

John Schmitt