Clocking will show consumption. If you think the pressure is off check it!!
Its most likely very close. If you have had an annual service a
REAL HVAC guy would have checked the pressure and set anyway.
Clock will tell you what approxmate size.. But suppose its under/over for the building. Do a heat loss study. No use spending more on energy than is
necessary. Too big and too small is both a problem. Just do it right the first time!!
One HUGE company here bought a rail car of furnaces many years ago and
it didnt make any difference on the size/ sq ftage.. EVEY customer got
a 100,000 BTU install!! Whee.........course gas was 1/2 the price or less
back then.............