Issues Brexiters really care about and will likely get
2. Control of our own laws
5. Control of immigration. Net migration seems to now be about 300k to the UK each year. The Tories promised to bring it below 100K. If immigration drops below 100k that probably means the people who voted to leave the EU have the immigration control they want.
Things that are not measurable
1. Certainty wherever possible
11. Co-operation on crime, terrorism and foreign affairs
12. A phased approach, delivering a smooth, orderly Brexit
Things they had before Brexit
4. Maintaining the common travel area with Ireland
6. Rights for EU nationals in Britain and British nationals in the EU
8. Free trade with European markets
Measurable things (I think they won't get)
3. Strengthening the United Kingdom
7. Enhancing rights for workers
9. New trade agreements with other countries. This probably breaks down to improved economy. So measures of the economic trade could be used to measure this one.
10. A leading role in science and innovation
I am willing to pick measurable metrics on these last four. % of people in Scotland who want independance. Where UK stands in global metrics of workers rights. Patents or and journal paper outputs and their are other metrics of countries innovation. University league tables are another possible metric for example.
Trade agreements are mainly about the economy. Inflation, consumer debt, Sterlings value, GDP growth, export growth are all useful metrics.
I can't think of an obvious metric that shows making your own laws in Parliament has been a good idea. But there are 8 other objectives May wants that are measurable. And general economic metrics most people accept as important.
With at least 10 things to measure to decide if Brexit is going well or badly I think it is reasonable for Leavers and Remainers to define what they would see as success for Brexit. This wont take into account big downside economic or military risks. Or peoples happiness at increased national sovereignty, though national happiness metrics might work.
But you can measure some things people say are important so why not define metrics of what would mean Brexit was a success?