Sir, tell me what is the indirect tunnel Oven standard for suction pressure.
Are you referring to an Indirect Radiant (cyclotherm) oven or an Indirect Convection oven?
Does your question about suction refer to the extraction of air and moisture from the baking chambers?
Normally for indirect fire oven the suction is measured at the far end of the combustion chamber where a inspection cum suction measuring port given. Here only the suction monitoring equipment kept to see that if suction is not there then it will trip the burner that means in case of circulation blower is tripped the burner will get tripped.
I think you are mentioning this only.
Here the suction pressure should be 5 to 10mm water column only. If more than that will reduce combustion efficiency and less than that will spoil the combustion chamber.
As far as I know since oven will be under positive pressure only as heat and steam is circulated as it will suck the steam and flu gas from tubes.
On regular basis you can open the inspection mirror and keep some thread or small piece of cloth to check the suction by gut feeling and ensure that you are running a health heat circulation system.
Inside the oven atmosphere ( outside the tubes) there should be negative pressure to facilitate the out coming moisture and other leavening agents from biscuit dough, flow uninterrupted. These will help in lifting the dough piece and forming a desirable structure.