Yup, you guys got it right.
Electric companies charge you by the kWh (kilowatt hour) which is essentially a unit of energy that equals a 1,000W device running for one hour (it's a measure of the wattage consumed by the device over a period of time of one hour).
So to get the kWh used by an FX6 that draws 43 watts you would divide 43 by 1000 = .043kWh (.043 Killowatts is how much energy the FX6 draws per hour).
To determine how many kWh the filter uses per day, you multiply .043 by 24 hours = 1.032kWh
Since we now know how many kWhrs the filter uses per day (1.032), we can then multiply that by 30 days to get us the amount of energy used over a month 1.032 x 30 = 30.96kWhrs.
To figure out how much the electric company will charge you per month, multiply 30.96kWhrs by whatever their rate is. The national average is $0.11/kWh, so if you're paying the national average you multiply 30.96kwh x $0.11 = $3.04 per month.
Hope that helps.