Calculate water remaining in espresso machine

[simons-plugins]

Much like showing weight of beans remaining, I'd love to be able to say I added 1.8 litres of water to my machine and watch it show estimated water in it, or even number of shots since filled, so I can estimate water remaining and get indication when I should refill ahead of it running out. Maybe using water out weight plus a user variable to account for retained water in bed etc which would be machine specific.