Mark
The the fluid expands from agitation and fluid temperature. So some form of 'ventilation' or other expansion reserve capacity is mandatory. That could be overflow recovery (think radiator/recovery tank) or a generous air space above the 'cold/at rest' fluid level. For example - outdrives are totally sealed no venting because they are typically submerged in seawater - thus there is pretty good empty headroom for fluid expansion.
So If it were me I would fill to minimum level, use, and during use keep cleaning up the overflow as it occurs. Do not add any fluid thereafter and see if the overflow rate declines and eventually stops. My bet is it will. More specifically is make/or obtain a screw and use that in place of the dipstick. The new screw is drilled through the top with a press on tube leading to a plastic bottle. That way you can see what is expelled and whether it stops without the mess you are suffering now.
Think of it this way (hypothetically of course) how would it behave if the measuring stick was too short. That would mean an overfill but you would have never know based on the stick. In motor builds I always use a measured quantity of fluid and then see if the stick marks match up - sometimes they don't even if they are factory issue.
Would not worry about too much fluid expelling - you not suffering from too little fluid.
Charles
Edit : Forgot - I assume yours is the #60 Drive and there should be a breather next to the stick - see that it is clear and operative . I would still stage the experiment I outline above
