When it comes to progress, what counts as 'sufficient'?

“The key issue for ‘sufficient progress’ will be money,” said Charles Grant, director of the Centre for European Reform think-tank. “The EU and the U.K. will have to agree on a methodology for calculating the amount that the UK must pay.” Such a methodology would have to be “detailed enough to reassure the EU … but vague enough so that journalists cannot easily work out a direct figure,” Grant added. EU estimates of this figure range from €65 billion net to €100 billion.