A quick google search came up with this chart:
from this PDF
Lets use the above parallel example of 18 parallel lines and using 1/2" pex for everything. It is going to have to flow through the tee as a run for 7 fittings, and as a branch for 2 fittings. That gives us an effective additional length of:
16 * 2.2 = 35.2ft
2 * 10.4 = 20.8ft
Or a total of 56 extra feet.
56 extra feet at 1 gpm gives us an additional pressure head of 2.3 inches of water. That would bring the total pressure head up to just about 2.9 inches of water even. Still quite a bit less than the 6.6 of the standard setup.