Somewhere in the service lore of the particular model may be something about having certain cylinders in certain positions when checking hydro level. I have to wonder if the sight glass is mostly for maintaining a minimum vs avoiding too much 'in tank'. The reservoir is more like a car/truck's cooling system thani t's lubricating system. Are there symptoms when going too far above sight-glass-level, or might this be like a 'best when used by' date on table salt?
In a hydraulic system it's only the reservoir that can ever be 'over-full', unlike an engine where high oil levels (common 'wet sump' setup we drive daily) can be stirred to foaming by crankshaft revolutions thrashing it. Machinery fluid levels are never fixed. Any & all fluids will expand when heated from 'room' to 'operating' temp. That's why we don't fill things 'to the cap', lest they spill out while running, and why there's typically an airspace at the top of a tank or reservoir. What I'm getting at is that if hydro isn't spewing from a breather or filler neck, is it really 'too full' as is for now and need to be drained lower?
I don't wish to steer the OP to some hocus-pocus approach, but sometimes 'the book' is so specific for the sake of 'the book'. Do we use only JD, NH, or Kubota-branded fluids, too? Hmmm ...