FOr 46 G tank and new. How about inexpensive Aquarium systems test kit for both. If serious, look into Salifert.
I have used Refractometerown by others but never own one myself.
I believe hydrometer
s I used was accurate enough since SG of natural sea water does shift slightly depending on their weather. I dont believe in precise since most of tools are actually not a professional grade to begin with and even natural sw is not same 24/7/365.
I am not saying not to buy one, just a thought!
What ever you do dont go crazy trying to match so called perfect levels.
IMO, no such thing. Close is enough.
It is somewhat marketing strageies developed by manufacturers also, I feel..
ALways perform gradual changes, giving time for livestock to adjust.
If possible, have small reserve running 24/7/365 in case of emergency.
I know some one who tried to save substatial amt of money by buying someone's homemade chiller for reef, against my advice. Wiped out almost all of livestock because it was made out of copper tubing. I assisted in saving some or should I say parts of some corals from his well stocked 90G reef. LR were garbage for reef once exposed to high copper level, unless used for FO tank provided level were controlled.
My point, dont make decisions abruptly. Always reason before action.
Good Luck and hope all goes well.