Hey Mouse, I did a little bit of googling and I found this vid that might be helpful...

https://www.youtube.com/watch?v=vxepmtjmilQ

In the first couple of minutes, he shows how to make a new partition in Windows (something I'd forgotten you could do). I think it'll be much simpler if you do it that way, as you'll know exactly where to put Mint. You can skip the BIOS bit in the vid, since you've figured that out already. At 6.00mins, he shows the partitioning bit, and it's much more straightforward when you have a chunk already marked 'free space'. Does that help to simplify things a little?

Towards the end, he uses another utility that I hadn't heard of before which switches the bootloader back to the Windows one, but with Linux added manually. You might want to do this too, though it isn't necessary. Do you remember before I'd said about some people wiping their Linux partition then their system wouldn't start? What he does here would get around that, so if you change your mind and want to reclaim the space, you can easily just reformat the partition and all the Linux stuff would be gone. However, it's not strictly necessary if you plan to keep Mint. Linux installs its own bootloader which will recognise both Linux and Windows installations.

About sizing, I just checked my installation properly tonight and it seems to have swollen up to 10gb. That's for my operating system and all programs. My own files are separate. So if you went for 50gb, that would leave at least 40gb for personal data files (assuming our installations are broadly similar - allow a gb or two either way!)

The odd numbers are because there are 1024mb in a gb. Just round everything off. It doesn't really matter.