I am glad you made the decision to install to sda. It is easier, but also it is the best configuration, in my opinion.
Boot your Mint DVD or usb
Open the gparted program
Delete the data partition (since you do have a complete backup) - right-click on the partition and choose Delete - or like this http://www.dedoimedo.com/images/computers/2009/gparted-menu-partition.jpg
You will then have all unused space (unallocated space)
Right-click on the unused space, choose New, then pull the right side handle to the left until you have between 16000 and 20000 MB as the new size (16 to 20GB). The number should be divisible by 4 for best results - like this -
File system: ext4
Repeat the process to create a swap partition of a little more than 8GB
File system: linux-swap
Create your data partition or partitions with the same procedure. Since you have 86GB of data that you want to share with Windows, first create a partition to use for /home for Mint - about 50 to 100GB (or your choice), Format: ext4.
Then use the rest for your Windows data partition - Format: NTFS
Apply the changes by clicking the green checkmark on the toolbar.
Now you can exit the gparted program.
Start the installation program and choose the "Something Else" option. Double-click on sda1.
You will get this pop-up window where you choose Format to ext4, Mount point /
Same procedure for the swap partition. Double-click on sda2 - Use as swap, Mount point swap
Same procedure for the Mint /home partition. Double-click on sda3 - Use as ext4, Mount point /home
Leave the default to install the bootloader to sda.
Complete the installation. Change your BIOS to boot sda first, and the option to start Mint or start Windows should be on the boot menu (GRUB)