Basing on the original derivation of field synergy principle, we propose two rigorous rules for its applications with respect to incompressible flow in conventional-scale duct: (1) the synergy angle should be evaluated in the thermal boundary layer for laminar flow; (2) the FSP analysis must be refined to the viscous sublayer for turbulent flow in the strict sense. These two rules are verified by two specific computational fluid dynamics cases of forced convection in two-parallel plates with irregular boundary condition in laminar and turbulent flow regimes, respectively. By making elaborate post-processing, the local synergy angles in the very near-wall region are clearly visualized. It is found that the patterns of synergy angle distribution in laminar boundary layer/turbulent viscous sublayer are able to accurately reflect the local heat transfer ability, which provides an opportunity to directly visualize the local variation of convective heat transfer coefficient. Quantitative analysis also shows that the local Nusselt number variations are in perfect agreement with the local synergy angle variations according to the rules we proposed. This work also can be regarded as a verification for field synergy principle from a new aspect.