The problem with such a simplification is that you may quickly add up total head losses higher than the dead head pressure of the pump. As skulemate stated, head loss through a fitting or hose is related to the flow rate. ie, a 90° elbow will have a head loss of 1 foot H2O only at a specific flow rate (not to mention elbow diameter). Fittings tend to have "equivalent lengths" that are proportional to their diameter. Since flow velocity is related to area (diameter^2 for round fittings), a given flow rate will have much lower head loss in a 1/2" fitting than in a 3/8" fitting. This becomes part of the balancing act where larger fittings result in higher flow, yet somewhat lower flow velocity and head loss.
It will provide an approximation, but could easily be off by a factor of two or more.
What is more accurate, though not much more difficult is to use tables relating head loss to flow rate and diameter. Even this has limits, however, as you'll be hard pressed to identify a head loss for radiators or blocks without actual testing.
If a water manometer is out, you could always use a mercury one.