I drove myself crazy last year when I got my minis Rocky and Cooper. Rocky looks like he is about Rosie's size. He is 34" tall about 8-9 yrs. and is very similar in stature to your Rosie. When I tried to calculate his weight, I also came up with 2 different figures, again, very similar weights to what you came up with for Rosie. Because I got my boys from a rescue I didn't know whether they came from a dry lot or green pasture, grain or no grain, so I was super nervous about how to feed them. I purchased hay nets and the above referenced hay pillows, because I was afraid to over feed them. I tried the hay nets and found them to be a pain and never tried the pillows (I still have 2 brand new ones in packaging). I decided rather than drive myself crazy with worry I'd just feed them the same as I do my 2 mini donkeys (and my previous mini who passed away), which is free grazing in the pasture during the day (just watch them in the spring when grass has a lot of sugar) and give them a slice or two of good quality hay during the night when they are stalled. All my minis also get 1/4 cup of low carb grain morning an night. The boys seem to be doing fine and their weight hasn't really changed one way or the other in the past year. I should add that they are also super active and run around constantly, so I'm sure that this helps keep their weight in check. Others on this site have way more real experience with minis than I do. Even though I've had horses in my back yard for over 30 years, I have much to learn and each animal is different. When trying to figure out what to do with my minis, I read what others experiences have been, compare and add them to my own experiences and come up with a plan for whatever the issue or task is. If I see the plan isn't working, I modify it until it works. So far it seems like you are doing great with Rosie. I look forward to your future adventures.