The Eirp or total system output is quite simple to measure.
It is essentially a mesurment of the TOTAL amount of power that leaves the radiating element minus the loses... (I.e coaxial length, connectors .... etc)
Example scenario.
Fcc permits 4 Watts for ptmp (34dbm)
1 Watt amplifier = 30 (dbm)
6 dbi antenna= 6
15ft of lmr 400 = -1
... You simply add all of the above, including the negative (same as subtracting in the latter) and you will get 35. Because you are 1 dbm over the legal limit, you have a few choices ... decrease amplification power, decrease size of antenna or increase length of antenna.... Well you get the drift.
The rules are a bit more lengthy and technical but the above should be helpful.
Regards.
Ps. do not get confused at you will see 2 Watts differs from 4 watts by only 3 db. In a perfect world, (2 watts = 33dbm which means 4 would equal 66dbm) but that isnt the case, rf power is not linear but rather logarithmic as a 6db increase would yeild 10x greater output power....
Further info ....
http://www.fab-corp.com/pages.php?pageid=1