15 to 20% of total is way too big for a Heads cut. Especially if you apply a 20-plater. Butterscotch tastes and flavors are caused by diacetyl. Not by definiation Heads compound related, but mostly caused by bacterial infections during the (early parts) of the fermentation process. More areation at the beginning (for better yeast propagation) or higher amounts of yeast may help solve the issue. My thinking is that you do have an off flavor (which you describe as "butterscotch") due to fermentation issues and that you may not have a cuts and/or concentration issue while distilling. Barley, especially malted barley, is very prone to bacterial infections.
Here's more info on diacetyl:
DiacetylDiacetyl is most often described as a butter or butterscotch flavor. Smell an unpopped bag of butter flavor microwave popcorn for a good example. It is desired to a degree in many ales, but in some styles (mainly lagers) and circumstances it is unwanted and may even take on rancid overtones. Diacetyl can be the result of the normal fermentation process or the result of a bacterial infection. Diacetyl is produced early in the fermentation cycle by the yeast and is gradually reassimilated towards the end of the fermentation. A brew that experiences a long lag time due to weak yeast or insufficient aeration will produce a lot of diacetyl before the main fermentation begins. In this case there is often more diacetyl than the yeast can consume at the end of fermentation and it can dominate the flavor of the beer.
I hope you find this information helpful.