# your current inputs
gas_m3 = pumps × rate × hours
= 8 × 0.25 × 8,760 = 17,520 m³
ch4_t = gas_m3 × CH₄_fraction × 0.668 kg/m³ ÷ 1000
= 17,520 × 0.90 × 0.668 ÷ 1000 = 10.53 t
co2e_t = ch4_t × GWP
= 10.53 × 28 = 294.9 t
Density. 0.668 kg/m³ is methane at standard conditions (15 °C, 101.325 kPa). Your measured rate should be corrected to the same basis.
Fuel gas. All vented supply gas is counted, not just its methane share. That's the gas you stop paying to throw away.
What it ignores. Pneumatic controllers, intermittent bleed devices, and any rate change with supply pressure or temperature. Measure if you can; it's the only number that holds up.