Either you estimate the heat transfer loss and include it on the balance of energy equation that is used to find the flame temperature or you measure the flame temperature.
(you; estimate; the heat transfer loss) (energy equation; is used; to find the flame temperature or you measure the flame temperature) (energy equation; to find; the flame temperature)
you energy equation energy equation
estimate is used to find
the heat transfer loss
to find the flame temperature or you measure the flame temperature
the flame temperature