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Do a search of all forums and all dates for calcium and you'll get a couple of pages of posts about this topic. Other search words would be 'filled tires', 'wheel weights', etc.
Both rear wheels weights and a weight box are removable. Since you mention eventually getting a mid mount mower, you might want to remove the weight to cut the grass. Others have filled their tires with windshield washer fluid or water, depending if you live in an area that experiences winter. Your profile is not filled in, so I don't know which would work for you.
The 'problem' with a weight box is that it has to be removed to use any 3ph device.
As with most topics on TBN, how best to get rear wheel weight is a personal decision. My opinion...I'd rather be able to remove the weight when/if I want to. If I was sold on filled tires, I would not use calcium. It's corrosive, if it leaks it kills whatever it touches, typically only a dealer can load it. I'd vote for windshield washer fluid or water (depending upon climate) which is something you can do on your own. Beet juice is another dealer installed solution.
I use wheel weights that consist of a bracket made by another TBNer on which I installed standard weight lifting weights, 150 pounds on each wheel (max recommended by NH for my tractor). The commercial version of a weight bracket is EZ Weights. Rear wheel weights, IMHO, are the best. Why? They can be removed if needed (filled tires are permanent) and do not preclude using 3ph attachments as a weight box does. Having said this, how you plan on using your tractor will determine which solution works best for you.