There are handy tools available for doing calculations like this, for example coinwarz.com/calculators/litecoin-mining-calculator/ .
But to help you out, I will do it for you.
You start by looking at the specifications of your specific miner, which can be found here:
https://shop.bitmain.com/overview.htm?name=antminer_l3_litecoin_asic_scrypt_miner .
This miner can provide a hash rate of 504 MH/s tolerance of +/- 5% but let's just assume that it has a constant hash rate of 504MH/s, then you need to fill down this value into the Hash Rate (kH/s) box which is 504*1000kH/s so 504000. Now add the wattage which is 800 watt with a tolerance of 10%. Always go with the worst value, so fill in 880W into the power box. Now fill down your costs for electricity per kW/h, which can be found on your electricity bill. Add this to the Power Cost box in dollars. Now do the calculation, this will be the profit you can make at the current difficulty. Keep in mind this value changes every few days. My calculation gave me a result of $352.61 per month, excluding electricity and hardware costs.
