If you like to use a previous checkout page that you have created from a previous Mass Rank Promotion for a new promotion or a promotion in the future, you can do that by going to:
Left Side Menu Bar > More Features > Checkout Pages
Under "Checkout Pages" page, scroll down to "General" section then click "Edit Checkout Page" button.
Under "Add / Edit Checkout Page" page, scroll down to "General Settings" until you find "Associate to a Rank Promotion?" section.
Note: Make sure you create your rank promotion first. Once it's created, go to the checkout page you want to use for that mass rank promotion.