sugar - just means common old household white sugar (Sucrose), its used as the base unit for most brewing calculation because it's very pure, very available, very cheap and doesn't have any water either in it or chemically attached to it.
So it contributes all of its weight to the solution, unlike Dextrose (Dex, Dextrose Monohydrate) which has a water molecule so its only about 91% "sugar" and 9% water (we have to allow for the water), same for DME (Dry Malt Extract) which isn't totally dry(3-4% water), or LME (Liquid Malt Extract) that is about 20% water.
~350g of sugar would add enough mass to bring you up to your target 1.045 (if that's your target)
~385g of Dextrose will give you the same gravity
~363h of DME
~437g of LME
Would all get you to about the same 1.045.
Sounds like there is a fair chance the FWK was a bit low in gravity, it can happen, sometimes the first one out has a bit of rinse water in the plumbing from the kettle to the cube.
Hope that makes sense Mark