What is the electron energy that will give dose coverage $>93\%$ of the prescription dose to a target volume that extends from $0\mathrm{.}5$ cm to $2\mathrm{.}5$ cm below the skin surface and has an $8$ mm bolus placed on the skin, while minimizing the dose to an underlying avoidance structure at $4\mathrm{.}5$ cm from the skin surface to $<50\%$ of the prescribed dose?