As I understand it, 'modern' unleaded petrol burns slower than the old lead stuff, so to get the most bang for your buck timing should be 6 degrees before TDC, however as it has been stated above that's for a new engine without any wear. Use 6BTDC as a starting point then fine tune by ear to get the best from your engine. Mark your dizzy before you move anything so you can reset back at the start if you muck it up.