Dan S. MacAbre wrote:

Thanks everyone for the advice. We don't have kitchen scales, since
both the missus and I believe that 'the spoonful' is an accurate enough
measure. I do have digital luggage scales (wouldn't dare go on holiday
without one, these days), which just might be accurate enough, but I'm
not going to bet a huge amount of money on it.

Presumably it came in an e.g. 5kg bag? Squish that into a rectangular
shape, measure the sides, and you can calculate the volume. Then just do a
bit of long division to work out the volume per dose, and find your little
measuring thing from the soap. That should be calibrated in volumes, and
if not it is relatively trivial to calculate from pi*r^2*h