I have Cannondale Si compression plug with flat top cap and I am using a Bontragar Blender Pro stem on my CADD 13. I wondered how I should locate my compression plug within the carbon steerer tube of my fork?
With a carbon steerer tube, you generally want to have the compression plug placed within the area where the stem would clamp. The general idea is that the clamping force of the stem has a solid object (the plug) inside the steerer tube to support the carbon steerer tube. You don't want it too low, otherwise the clamping force would not be supported on the inside of the steerer tube. You also don't want it too high (if the steerer tube extends above the stem with added spacers), for the same reason and in addition you don't want the top cap to "bottom out" on the compression plug (which could prevent the bearing preload adjustment from being effective).
If you have any other documentation for the bike/stem/fork specifically, you would follow those instructions first prior to any of the generalized guidance just stated.