He earns ten times the average good wage in Croatia. So I took the average good British wage £2,500 to £3000 per month and X by ten to give Brits an estimation of his standard of living.
Then that should have been included rather than claiming it as his income.
But it doesn't really work like that. While earnings are different in various countries in absolute terms, goods are often priced to sell in different areas. That's why everyone has TVs and refrigerators whatever country they live in ... they pay a lot less for them. Costs for housing and other things will also be lower to match.
There are sites that will compare actual earning power in different countries, often for people emigrating. Things can often be cheaper than the direct currency conversion to account for the income differences.
https://www.expatistan.com/price/tv/zagreb-croatia
Of course if you are living in a low-cost country and earning more prime rate income remotely, then you will be on a much better standard of living which may actually make it more like earning a lot more.
The bottom line is you probably get a more for you money in Croatia than in the UK.