How do I correctly measure this basement window for a replacement?
Foundation windows are a difficult item to order replacements for. The mounting and track is different for most manufactures. Since you can't easily change the steel frame that is embedded in the concrete, you need the right window glass frame and size.  My advise would be to remove the glass sashes and bring them with you to your supplier and see if they can identify the manufacturer. they can measure them and order the right items. The other option would be to take the glass and removable frame to a glass shop and have them reconditioned.