The breakdown electrical field for SF6+N2 mixtures was studied in 'A Study on Dielectric Strength and Insulation Property of SF6/N2 Mixtures for GIS' by Su-Youl Woo et al. (2012). They came up with the formulas for breakdown E-field (p.u.) being

with pgas being the gas pressure.

The dielectric strength is relative to air with dry air having a dielectric strength of 3 kV/mm and SF6 one of <8 kV/mm.

Symbol
$\epsilon_{\mathrm{gas}}$
Unit
-
Choices
IdGasDielectric constantDielectric strength
Airdry air1.0005361.0
N2N21.000581.15
SF6SF61.0022.5
CO2CO21.0009210.95