To determine the extent to which a company's capital structure should rely on hybrid bonds, we need to consider the cost of capital and standard corporate finance principles. 

Hybrid bonds are subordinated debt instruments that carry higher yields compared to standard senior debt due to their increased risk and equity-like features (e.g., coupon deferral). The provided market data explicitly shows a significant Subordinated-Senior (SUB-SEN) Delta (e.g., 2.295% in 2022 for EUR Non-Financial IG), highlighting the persistent premium a company must pay to issue hybrid bonds over conventional senior debt.

REN - Redes Energéticas Nacionais operates as a regulated utility with highly stable and predictable cash flows. Companies with such robust earnings profiles can generally sustain a substantial amount of cheaper, standard senior debt and standard equity without needing to utilize expensive hybrid debt to artificially bolster credit metrics. While utility companies occasionally use hybrid bonds as a temporary mechanism to defend their credit ratings during major acquisition phases or heavy CapEx cycles, an optimal, long-term theoretical capital structure does not fundamentally *rely* on hybrid instruments because of their excess cost compared to standard senior unsecured borrowing. 

Given the structural costs and the financial stability of a transmission system operator like REN, its core capital structure should optimally rely purely on traditional debt and equity, avoiding the premium associated with hybrid subordination. Therefore, the optimal reliance on hybrid bonds is 0%.

<answer>0%</answer>