celsius is more accurate anyway since that's what the motherboard measures the temps in. if you view it as fahrenheit, the program just converts it using the (9/5)C+32 formula.
i apply a small dab on the core and simply tap around on the core with the tip of the as3 syringe until it's covered. i don't worry about it being flat. then i put a tiny dab on the contact patch of the heatsink and rub around clockwise and counter-clockwise with a palstic bag like 2zzColt said. even if your heatsink appears to have a smooth finish on the bottom, this will help get it in the microscopic valleys and holes that exist.
my athlon 1400 puts out 78watts of heat and my temps never go above 45 in a quiet case with only one exhaust fan and power supply moving air, and a silent 60mm fan on the cpu. of course, having a good heatsink helps.