How can I calculate overall how much power my circuit is using per hour
But now im struggling to see how I convert this value into a meaningful value. Given my circuit is running at 5 volts I know that drawing 50 milliamps is going to mean im using 0.25 watts.


Correct.


  Do I then multiply this by 73.2? to figure out how many wattseconds im using?


Yes. (Note that Watt-seconds are also called Joules)


  Also from that can I then work out how many milliamp hours this is?


To get amp-seconds divide by the voltage again; to get milliamp-seconds multiply that by 1000; to get milliamp-hours divide that by 3600.


  The reason im trying to work this out is so that I can spec a battery. I think im close but im getting a bit confused.


If your battery isn't 5V I'm guessing you have some sort of voltage converter in-between the data logger and the battery. This calculation will tell you the amount of milliamp-hours drawn from the converter, not from the battery. (If it's a linear regulator they will be the same; if it's a switching regulator they will not be)