this is tricky
There's programming limitations in the program where for "your" "specific" "situation" you are not being asked the right question at the right time. Basically, you're asked "Percentage of time the property was rental during the year (e.g.;80%)" That's the wrong question for *your* situation. The question should be asking percentage of floor space. Not percentage of time. But if you treat the unit for what it is (a physically separate structure that is single family housing) you won't have to deal with remembering this every year.
Overall, because the ADU is new construction, I recommend you treat the ADU as it's own physically separate single family rental unit. It will make your life simpler down the road if/when you sell the property. In other words, using your numbers the property is it's own single family unit with it's cost basis being the cost of construction, plus 40% of the land value. Everything is 100% business use.
Then the only math you have to do is 40% of the property taxes, mortgage interest and property insurance. That's it.
Get the utilities metered separately. Otherwise, there are two possible and acceptable ways to split the utilities. It can get complicated on this front with utilities if you have periods of personal use/vacancy. But not an issue of the utilities are metered separately.
