Just tighten it firmly then to the next notch on the castle nut. Do not use an impact to install parts on an aluminum spindle (you should not be using one on these types of parts anyway). The spindle can crack easily with the force of the impacts before you realize what happened.
I did a van a couple of years ago that a chain tire store had replaced a tie rod on and did an alignment, they cracked the friggin spindle and the A hole put a big washer under the nut and said nothing, they turned out a job with potentially fatal consequences.
Make a long story short it broke out luckily when the guy was pulling in his driveway.