here are some answers I provided in 2007 to a similar question:
here are some references that are useful for determining the flow unbalance in manifolds/ headers ( as I listed in a earlier thread):
Bajura, RA "A model for flow distr...: trans ASME 98-A-4,473-479 Oct 76
Acrivos, A Chem Eng Sci V10 p112 1959
Keffer, JF J Fluid Mech 15 p 481 & , 1963
Bajura, RA, Jones, EH J trans ASME V93 ser A no 1 pp 7-12 Jan 71
Ahn, H KSME Int J 12(1):87-95 feb 98
Hager,WH P I Mech Eng C-J Mech 201(6):439-448 1987
Greskovi, EJ , Obara, J T Ind Eng chem proc DD 7(4):593 1968
A science citation search of newer articles that reference these papers may provide more modern techniques, such as using FLUENT etc 3D CFD to simulate the flow in the headers.
As I recall, some of the older correlations were based on the assumption that the header internal design was formed using "set-thru" tube welds, which led to a higher internal header friction pressure drop then the now normal design of a set-in flush weld of tubes to header.
Another detail to note is that the headers are often drilled with smaller holes than the attached piping, to allow fit-up if a set-on stub weld is used ( ie ASME sect I fig "Z" weld).So, it isnot always corect to assume the header drilled holes equals the attached piping ID.