To fill the tranny I use a "Nalgene" plastic bottle with a pointy top, marked at 0.3 Liter (the HB50 fluid volume) with vinyl tubing zip-tied onto the spout. Fill to the full mark, work the tube into the tranny, tip the bottle up, and let 'er rip. No funnels, no muss, no fuss, no runs, no drips, no errors.
This is the type filler bottle - a Nalgene lab dropper-bottle:
Or not quite as friendly, use a wash-down type bottle and squeeze it standing upright
(or cut the tube off right below the cap):
When I used to be on the hard during winter, I'd change the fluid in Fall and refill to the top (overkill to prevent moisture from accumulating?)
In spring I'd use this trick to get the fluid down to the correct level.
I have a battery filler with the tube marked w/ a zip-tie at the length of the tranny dipstick above its "full" mark (i.e., the bottom of the tube to the zip-tie distance = the amount of air in the tranny above the full level):
In Spring I'd squeeze and insert the battery filler to the zip-tie mark and evacuate the fluid (repeat if needed) down to the bottom of the tube. This "automatically" leaves the tranny fluid at the correct (full mark) level.
The same method can be used to add fluid between changes (or if overfilled when changing) -- overfill just a titch and suck it down to the full level (instead of using of the "add a little and check it" guess routine.)
Note: The fluid will make the two parts slippery, so put a tight zip-tie around the rubber bulb to hold it onto the tube or Gorilla Glue them together.