Ideally, you would use the definition of fast for Australian Catholics.
I have previously heard that it is two snack meals, and a small main meal, however I can’t find that on-line.
US Catholics have
this [USCCB]…
Q. I understand that Catholics ages 18 to 59 should fast on Ash Wednesday and on Good Friday, but what exactly are the rules for these fasts?
A. Fasting on these days means we can have
only one full, meatless meal. Some food can be taken at the other regular meal times if necessary, but combined they should be less than a full meal. Liquids are allowed at any time, but no solid food should be consumed between meals.
Australian Catholics might use
this [Archdiocese of Canberra and Goulburn]:
Catholics who are over the age of 18 and under 60 are required to fast on Ash Wednesday and Good Friday, which means
to have smaller meals during the day.
.
I would go with the US definition, as it seems a bit stricter and closer to what I have traditionally heard in Australia.