Taking a flue draught reading is not easy when trying to do once an inset appliance has been installed. There is no direct access to the flue pipe.
If the installer has fitted a sweeping access door somewhere, into the chimney system , then this could be utilised for placing the probe of the draught meter.
The only other way to get an approximation of the draught is to put the probe of the draught meter into the primary air opening where the control paddles are.
To do this you will have to partially dismantle the air control assembly and remove the primary air paddle (left hand side).
Upon refitting the control assembly make sure the probe of the draught meter is located into the primary air opening on the base of the appliance.
Then light the appliance and allow a small fire to burn and record the reading.