If you're looking for a large site migration to go without a hitch you're going to need a robust and methodical way of working through your redirects - no matter how big the site.
Migrations are the make of break point of any site process because you have, in theory, everything to lose if it is done wrong.
In this presentation Chris will run through the framework with which to approach redirect mapping of a new site process.
Rest of the blog writeup can be found here - https://strategiqmarketing.co.uk/blog/htaccess-hell-brightonseo-2016/