When you are adding Low Vis hydro fluid, add no more than 1 quart at a time. I use the 5 gallon buckets for Hydro fluid and I picked up a cheap plastic kitchen measuring pitcher from the Dollar store, in which I pour all fluid to add to the tractor. The rear hydro will go from not appearing on the stick to being overfull very quickly. That's why add no more than 1 quart and watch for it to reach the dip stick. Once it appears, the fluid level will be reached shortly thereafter.
Keep in mind the dip stick on the rear end assumes the tractor is on level ground, etc.
Do yourself a favor and pick up a jar of the hydro fluid red dye when at Deere getting your low viscosity fluid. The red dye makes reading the dipstick easier and it also makes seeing any leaks or drips easily identified.
John Deere part number MT3668 is a very small jar which will be half full of a red dye concentrate. The small jar will treat a 5 gallon bucket or the entire tractor hydro system. Add it to the tractor when adding fluid and pick up a spare bottle to treat the new 5 gallon bucket as well. It should be about $7 to $9 for the dye.
Note, this is the RED color dye, not the Infrared tracing dye which is also sold for tracing leaks, etc. It makes reading the dip stick much easier and it also helps significantly for those machines which use a sight glass on the hydro housing.