Consider a long cylinder $C = D \times (-L,L) \subset \mathbf{R}^3$, with heat applied to its horizontal boundary according to $\varphi$ and perfectly insulated ends. The steady state $u: C \to \mathbf{R}$ representing the temperature is the solution of the system \begin{equation} \begin{cases} \Delta u = 0 \quad \text{in $C$} \\ u = \varphi \quad \text{on $\partial D \times (-L,L)$} \\ \frac{\partial u}{\partial \nu} = 0 \quad \text{ on $D \times \{-L,L\}$}. \end{cases} \end{equation}

In heuristic terms, 'most important' for the values of $u$ on $D \times \{ 0 \}$ are the boundary values with similar height, say $\partial D \times (-1/10,1/10)$. Heat applied 'near the ends' of the cylinder by changing $\varphi$ on $\partial D \times (L-2/10,L)$ (for example) are still 'felt' by $u$ on $D \times \{ 0 \}$, but 'much less'.

Question. I am looking for references where estimates specific to this setting are established. Does somebody have a tip? In concrete terms I am hoping for bounds that quantify the 'waning influence' of $\varphi$ on $u$ with the height.