Farid,
I'd suggest you to follow advices as received from Harvey and Ankur - these are right on the spot. One additional comment I have is that all rules of thumb might become invalid in brownfield projects where you are forced to engineer a solution that must fit between given constraints. Identifying constraints that can be (and should be) changed, and those that cannot be changed, will likely lead you towards successful design and later, operation.
Having excessive pressure drop in Instrument Air piping will always result in two things:
1) Wasted horsepower of Instrument Air compressors (= $$$);
2) Inability to meet Instrument Air demand for remote users, which can ultimately lead to unscheduled shutdowns/trips.
If there is 2 bar pressure drop between the air dryer discharge and the most remote IA user, at the maximum IA demand flow, I would change the piping size. You can easily demonstrate how this works by calculating incremental Opex of the IA compressor versus cost of the new piping if you design it for maximum 50 kPa pressure drop, and you do this over the design life of the plant. I am sure that you can easily prove how expensive it is to pump air through undersized network. This simple economical calculation would serve as the basis for replacement of undersized piping, if the case.
So, rather than being tied to any specific "rule of thumb", you need to look at the system in question and make all necessary observations required for proper engineering design. If I remember well, your project was about design and installation of additional IA compressor in an existing plant. What if (I am asking this hypothetically) the bottleneck in IA supply is not caused by machines, but by an undersized piping network? Installing additional compressor will not help much in that situation. It is the same as if you are having an orifice in the piping and you keep adding more compressors upstream to push more flow, whereas it could be that you simply need to increase the orifice size and the flow would simply increase by itself.