Although rectal admin is much faster in it's onset, 15 mins might be jumping the gun a bit. I'd say 20-25 mins should be a much better indicator (although this reduces the number of times you can add to the original dose). After an hour, additional doses are not going to do anything but extend the length of the trip, so if you haven't got where you wanted with the 2nd additional dose (3rd dose including the original starting dose), you'll just habve to wait until another time.
As for the doses, 8mg rectally would be equivalent to about 16mg orally, but as it comes on quicker, this might be a tad uncomfortable. Maybe better if you went, 0:00-6mg; t+00:20 5mg t+00:45 2-4mg as this will reduce the body load (if you usually suffer from body load or onset anxiety with oral use, be prepared for possible unpleasantness from rectal dosing. Don't go for the IM route with this compound as if it's getting too much you can always wash out your colon with water to prevent any more drug getting into your system - this isn't an option with IM.
If possible have some benzos (fast avting pref, like Xanax) on standby if using such a fast route of administration as if things go sideways with this drug via this route, it can become extremely overwhelming in a matter of minutes and happen so fast that a trip sitter has difficulty being able to talk you down into a calmer space.
At all times, proceed with the utmost caution if you decide to use this method
But I've also found that sensation in the stomach area can be amplified in weird ways by 2C-C, which can be mistakenly interpreted as nausea. My stomach and I actually came to a mutual agreement when we were on this substance once - something along the lines of "I was more than happy to help it expel its contents, but all it really wanted was my permission as there was nothing to give." Since then I have enjoyed a freshly delivered pizza and red wine after coming off of the peak of 2C-C. 
