I think I have the answer and I'm a bit annoyed with myself for not thinking of this earlier.
If we know the strength of the sugar solution in degrees brix (Bx) then we know what the solution is as a percentage weight for weight.(%W/W) I.e. if the solution was 10Bx it would be 10 grams of sucrose in 100ml of water, therefore 10% W/W. Once we know %W/W we're half way there.
Hypothetically, if you had 1
kg of candy syrup at 50Bx it would consist of 500g (therefore 500ml) water and 500g sucrose, but
not occupy a volume of 1 litre. Can you see where I'm going with this?
The key is weight.
You can calculate the dry (approx. 100%) sugar weight to wet conversion if you know the Bx using this formula:
Mw = Md/ww
Where: Mw = Mass of wet ingredient needed in kg
Md = Mass of dry ingredient in recipe, desired or whatever in kg
ww= % of sucrose in solution as weight
Eg: recipe for Belgian piswater calls for 375g candy sugar but you can only get/have syrup at 78Bx.
Applying formua: Mw = Md/ww
= 0.375/0.78
= 0.481kg syrup or 481grams
The key word is grams. Place a cup on your scales, tare them and then
weigh out 481 grams of syrup.
Going the other way (wet to dry) is a bit more complicated. You could use this formula:
Md = Mw X ww
= 0.481 X 0.78
= 275 grams dry sugar
Obviously you'd need to know the Bx of the candy sugar in the recipe or be able to take an educated guess and the recipe would need to show the weight of syrup in grams which is unlikely. In that case what you could do is make up a solution to the desired Bx, probably about 78Bx or 78%w/w and then add that in. Just be sure if making up 500ml or whatever to add your sugar to a smaller amount of water and dissolve, then add water up to your final amount, as indicated by GalBrew earlier in this thread.
Can't get 78g of sugar to dissolve in 22ml of water? Make it up to 39Bx and add twice as much.
If you need to convert to Bx from SG you can use this calculator:
http://www.brewersfriend.com/brix-converter/