Don't get too hung-up on the pedantic's of head-loss (friction loss) thru fittings etc.

In your case, pumping from your holding tank using the Grundfos manifold will result in an unmeasurable reduction in pressure.

Normally when calculating pipe / fitting losses you add an additional loss to your calculation for fittings, bends, elbow, valve etc based on an equivalent length of pipe -- for your Grundfos manifold if you added an extra 15ft of pipe to the calculation it will be meaningless in the overall scheme of things.