Substitute for demerara sugar in cookie recipe I have a recipe for oatmeal cookies that uses demerara sugar. While I do have some demerara sugar, the quantity remaining is approximately 1/3 of what is needed for the recipe. (In normal circumstances I'd just go buy more but...) 

What I do have is plenty of several other sugars: white caster sugar, golden caster sugar, light muscovado and dark muscovado.  (The best substitutes suggested by google are granulated brown sugars which unfortunately I don't have.)  

Is there any one or combination of these that could be used to make up the remaining 2/3 weight of sugar while limiting the effect on the end result? (The recipe has the sugar creamed into butter, if this makes a difference.)
Demerara sugar has a syrupy flavor which is between light muscovado sugar and granulated sugar, I would substitute a 50/50 mix of golden granulated and light muscovado sugar. As you have some demerara you could do a small amount of 50/50 mix and taste test, adjusting to get the right balance. 

You could also put in straight light muscovado if you wanted a richer result, or golden granulated if you like it slightly lighter, cookies are generally pretty flexible on this.