Safe dieting is to feed 1.5% of current weight or 2% of ideal weight, which ever is greater, then make adjustments as weight is lost.
a 36" mini, measured to the last hairs of the mane as minis are measured, rather than top of withers, is probably going to weight in the 300-325# range, maybe a little more, depending on build (the 37" gelding I had, looked best about 350#, and was too thin at 325#; he was weighed a lot, as he had PPID and I was tracking his weight to manage his diet and condition). While not ideal, a weight tape would help track his weight; in my experience they tend to weight 30-50# heavy for large minis, but my tape was older, so could be stretched out. [The weight formulas work decently for minis.]
Soaking hay to remove extra sugars... If you can find tested grass hay that is 10% or less sugar/starch combined, you do not need to soak; but it's not always easy to find tested hay, and it's not budget friendly to test yourself, if you do not buy 6-12 months worth at a time. Soak for 30 minutes in warm water or 60 minutes in cold water to remove sugars; more time just removes other nutrients and risks souring the hay. Most seem to have easiest time using a hay net, and dunking in a muck bucket of water, weighing it down with a brick, then dump the water where the horse can't reach it. [Being it's a mini, you won't need a full muck bucket of water, but use plenty of water, more water will help leach out more sugars, as the water doesn't become saturated with sugars as fast.]
I don't share this pictures often, but I bought Misty obese, and she went on a diet almost immediately. Not too long before I bought her she had been on a free choice alfalfa diet, the person I got her from had moved her to grass hay, but still free choice plus grazing. When I got her, I controlled her intake, and fed 2% of her bodyweight daily. She's a 38" mare, that typically weighs around 390# without being obese, but probably a little chunky. In the pics she was only a 2 year old (3 year old for the year later pic), so not filled out nor at mature build. Weight tape said she was like 440#, formula showed lower. I've kept some record of weight over the years, but not all of them, and no pics were taken on the day I took weights, so I don't have exact matches. [The person I bought from has been photoshopped out of the "fat" pic.] Her diet was measured grass hay (I can't remember how much I fed her, but it like 10# daily or a bit less, being she's a big girl) and a size appropriate serving of ration balancer (at that time, it was Progressive Nutrition Grass Advantage, but that product is now discontinued).