Use GParted Live.
The reason you're not able to use GParted to resize your partition is because it's mounted as
/, you can't edit a mounted partition, and you can't
umount it either as it is
/. That's why they made GParted Live.
Burn it to a CD or DVD using something like k3b or any other program and boot from it, choose live mode and it will prompt for a few entries while booting, just press enter at each one. If live mode hangs or anything, just choose "other modes" and then "safe mode".
You'll be met with GParted GUI on boot, where you can Resize your partition to clear up free space, there will be a warning but it's very safe, you might want to leave about 150GB on your Ubuntu partition just to be safe, also understand that GParted standards dictate 1MB = 1024 KB and not 1000 and it's the same for all other multiple units. After resizing there will be free unused space, which you can use for your installation, either format it using GParted or while you're installing Windows.
You might also want to leave some free space between partitions, just until you're sure you're satisfied with your space and partition table and then join it to any other partition. DO NOT leave free space lurking in between partitions on your drive, it could create read/write issues and shorten the drive life span, it's not really lethal but still not something you want to do.