try about a inch to start with and than lower or raise dependng on you.
When you fish i always feed a stick made up of crumb or GB. when a fish goes over all the little particles will float of the bottom and waft around irriatating the carps taste, this will undoubtledly attract fish but wont feed them. leaving ur pop-up to be taken (only food item there), try anything that wafts off the bottom and flavour it so its similar to hookbait.
The other way i use them is to pop them up about 1-1.5 cm and then feed whole bloilies and chops over the top, i think a low pop-up looks like a bottom bait to a 30-40lb carp but is easier for them to see and pick-up and has the ability of the hook hanging. Not sure why but it works well and gives you a massive edge. I caught all my latest fish (upto 37.9lb) on these methods and its strange the lake sees popups score 3 to 1 on bottom baits always has done, so it might help on your lake.
Also use a simple rig not a withy,swivel 360 mega hooker just somthing that flips the hook and catches well enough. hope this helps