I'm assuming you want to offset what you're lifting with the loader. What materials are you lifting?
What are you filling the ballast box with? I'd suggest using something that's easy to add and subtract from the box (steel scraps, sand, stone..., not concrete).
I'd go with your current 700# or so (or less), lift the heaviest thing you plan on moving with the loader and see how the back tires feel. Are they lifting? Do you have adequate traction without using 4WD?
I'd think your tractor should handle decent FEL loads without ballast at all.
