Correct pressure setting for gas valve for old Lennox furnace

[LikeABigFriedEgg]

I replaced the gas valve in my furnace a few years ago, and I don't know whether the gas pressure and air feed are adjusted to suit the furnace. It is a Lennox G8Q2-110M, manufactured April 1967. Old, I know, but I need to make it last a while longer. The valve is a Robertshaw 720-402:
https://www.robertshaw.com/Products/Heating/Gas-Valves/720-402/

The valve supposedly comes preset at 3.5 inches w.c.

But I don't know whether that is a good setting for the furnace. The flames seem mighty high---probably more than four inches. I don't have much idea about what is normal for that.

I can rig a manometer from a 1/8" NPT x 8mm barb and a 5/16" vinyl tube.

Should I change the pressure to something other than the valve's factory setting?

Thanks.

 

[LikeABigFriedEgg]

The furnace input is rated 110,000 BTU/hr.

That is about 108 cubic feet per hour.

What if I adjust the gas pressure so that I achieve a flow rate of 108 cu. ft./hr. as observed at the meter?