Before you go for this, do your homework. Hover over the achievement in the achievements list to see exactly which provinces are required, then open the decisions tab and hover over the Restore the Roman Empire decision — it highlights every eligible province on the map. You'll notice it includes provinces in Britain as well. If you didn't expand there early, Britain can become a serious problem at this stage. It's possible to ignore the British provinces entirely, but just remember you'll need 425 provinces total within the eligible region to compensate — again, check the decision tooltip to verify. Plan accordingly well before you think you're close.