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

Press quote (Politico)
05 September 2017

“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.