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Changing the calibration won't help you. Read "Measurement implications of ADC resolution at low current values" in Building Blocks.

What is the maximum current you want to measure, and what CT are you using? You might be able to improve the voltage seen by the ADC by changing the value of the burden resistor.

I'm using the sct-013-000. Since my appliance leakage current interrupter is only built for 40 Amps I need to measure from 0-40 Amps. I'm using a 3.3volt Arduino clone and a burden of 33ohms with a calibration value of about 55.
What would you suggest?

For a 3.3 V Arduino, you need a rms voltage across the burden resistor of about 1.1 V. The CT produces 50 mA per 100 A, therefore 20 mA at your maximum current of 40 A. So you need a burden resistor that develops 1.1 V @ 20 mA. 56 Ω will probably be OK. (It just eats a little way into the allowance for component tolerances, but use 51 Ω if you're happier.) That should lower the minimum you can measure to around 180 mA. The calibration constant should be 35.71.

Okay, so I've set up a new board with a 56 Ω resistor and your calibration value. However I'm confused why the power Usage doesn't rise by 3.8 Amps when I plug in my electric sandwich maker which is rated for 3.8 amps. It only rises by about 2.1 Amps.

I'm measuring inside my breaker box on the N (blue) wire. Most likely I'll have to replace that with 3 CTs (one for each phase), right?

This is the first time you mentioned a 3-phase supply. I think you need to study the theory of 3-phase: The neutral current is the vector sum, not the arithmetic sum, of the three phase currents. That means that if the currents in the three phases are equal, there should be, and you should measure, zero current in the neutral.

Does your sandwich toaster have a 3-phase supply? I very much doubt it. You say the current "rises" by 2.1 A, but what was it displaying before, and what is consuming that current?

I think it is quite likely that you are measuring noise, either current in adjacent conductors or digital noise from the Arduino itself. What do you measure with your CT in free space a long way from any current-carrying conductor?

Well, I'm assuming it is 3-phase since my electric stove is connected to it and there are 4 wires coming out of the wall. Here's a Picture of the breaker box cabling http://i.imgur.com/TBB65MW.jpg

Before, it was displaying about 0.6 - 0.7 Amps which is the idle power usage of my computer/NAS etc. running. I'm sure that isn't noise because when I turn off the computer it does fall for about 0.4. Also, the measured current changes appropriately when I turn on my washing machine, my stove or just a lamp.

You do indeed have a 3-phase supply, and you are measuring the neutral (imbalance) current. Therefore, unless all the loads are on the same phase, you cannot simply add and subtract the scalar values of current, because there might be a 120° phase difference between them even if they are well-behaved resistive loads. If there is an induction motor or a switched-mode power supply, the phase angle is anyone's guess and so is the resulting current.

You can only perform a meaningful check on the calibration by measuring the current in the phase conductor (brown, black or grey) or the corresponding neutral conductor of a single resistive load. Measuring the current in the combined neutral conductor is meaningless for this purpose.

My 2 other CTs just arrived. By measuring all 3 phases everything works like one would expect. Thanks for your quick and useful support.