Leave the butter in it's wrapper and carefully place it onto your microwave safe dish.
Place the dish into the microwave and close the door.
Run the microwave for 10 seconds at full power.
Open the door and feel the butter. This is where you need to use your judgement. My butter still felt relatively firm but not cold anymore and not actually warm either.
If the butter feels how I just described it, turn it over and nuke it for 10 more seconds.
If the butter feels a bit softer than I described above turn it over and microwave it for 5 more seconds.
If the butter feels firm but still a bit cold, microwave it for 15 more seconds.
After this you can carefully unwrap the butter and resume your baking.
After the first-or-so times you do this, you'll have an idea of exactly what to expect from your microwave. Then you'll be golden. In my experience, most microwaves are pretty consistent and I've personally had consistent results without our microwave (which isn't new but isn't too old either). But it's all about feeling the butter as you go.