You _can_ use guided partitioning as a guide. Use Windows to reduce the amount of space it takes on the disk. Use Windows tools to format the second half of the disk, or whatever to vfat. Boot Debian: use Debian to delete the vfat partition and create blank space: then use "use largest blank space" and guided partition. Debian will recognise there's another OS there and will insert the appropriate booting magic into a GPT formatted disk with UEFI. I have (at least) one laptop here with exactly that configuration.
If you're not sure, can I recommend Raphael Hertzog's Debian handbook - which has just been released for Debian 10. See
planet.debian.org recently. Download and read it in slower time.