You need a full time monitor to correctly get the percentage. The burden is the percentage of total beats that are PVCs (or PACs) is found by dividing the total PVCs beats for an extended time period by the total beats from that time period. So for example say your HR is 70/minute.
In a 24 hour period that would equal 100,800 beats. If your PVC beats = 20,160 that would be 20,160 divided by 100,800 beats which = 20%. A longer period on a monitor would give a better reading. Last spring I had an extended period of PACs. I wore a monitor for 2 weeks and the burden during that time was 24%. Ouch.