Draft Pressure Profile In Gas Cracking Furnace

Actually I'm doing one project regarding draft profile in a furnace. The furnace on which presently I'm working is handling C2 feed. The Id fan at the top of the furnace is designed for following specifications:
Capacity:2100 m3/hr.
del P offferd:130 mm of H2O
RPM:990
The draft at the bottom of the furnace is 14.8mm vacuum of H2O, at the arch section 5.5mm vacuum of H2O,near the damper 111.4mm vacuum of H2O and near the ID fan it is 140.5mm vacuum of H2O.The damper is presently 54.6 % open.
My 1st question is why the draft is getting reduced near the arch section and if it is so how the flue gas is flowing from the bottom to that section? I have fuel gas data, flue gas analysis data. Now I have to find out pressure at different location of the furnace regarding Id fan capacity. Now how can I incorporate that pressure increase in my calculation?
What are the data that I need basically to calculate this draft profile?