Yes, you can calculate by weight or by volume: 1 part salt to 4.5 parts water; (100-18=82. That's 18 parts salt and 82 parts water.
To find out the ratio divide 82 by 18 and you get approx 4.5 so the ratio is 4.5:1). That being said, large flakes of salt trap lots of air in them. I also hope you are using pure salt because some salts have anti caking agents added to them and there is also a % of other natural minerals.
The absolute best way to measure salinity is by using a salometer; an inexpensive idiot-proof device. It is essentially nothing more than a glass buoy with marks on its side that indicate the salt level based on how well it floats. (Because the more salt you have, the denser/thicker are the water and the effects of gravity are reduced). So, the more salt is in your brine, the higher it floats and the number you see is higher.
It's the same principle of floating on the dead sea (which is 18% salt as oppose to 3.5% in a normal ocean, where you don't float) or, if you have ever dunked your cheese in something not salty or light brine, you see that it sinks to the bottom immediately. At 16% salt, the brine is dense enough to float the cheese so its top side is just slightly below the water line. At 18% the top side of the cheese is just touching or slightly over the water line. At 24% half the cheese is immersed and the other half if above the water. Makes sense?
You can get those on eBay or Amazon for about $20. They are non chemical and non mechanical, they never need replacing or calibrating, but they are rather fragile. Here's my salometer in a pitcher of 18% brine:
