Tim+ submitted this idea :
It could also be interpreted to mean one fiftieth part in the final mix.

Hardly, that is not how it is written. Instead of writing 50:1 it could
be written as 50 parts to 1 part, or 50 measures to 1 measure.

If they meant that 50 was the total measure, they would write it as
49:1. The total volume is not mentioned and doesn't really matter,
unless space is limited in the mixing tank.