So, if you agree that the purpose of seed fermentation (a.k.a inoculum culture) is to scale-up biomass, then the correct key performance indicator is final specific growth rate.
To visualize final specific growth rate, plot biomass against time:
The cell density increases exponentially, which means on a log-scale, the curve becomes linear. The specific growth rate (μ) is the slope of the line. The final specific growth rate (μF) is the slope of all the points recorded in the last 24-hours prior to the end of the culture.
To compute the final specific growth rate, simply put datetime or culture duration in the first column, biomass in the second column, and the natural log of biomass in the third column:
In Excel, use the SLOPE function to compute the slope of the natural log of biomass:
Alternatively, if you don't want to bother with the third column:
This number has engineering units of inverse time (day-1). While this measure is somewhat hard to physically understand, we look towards the ln(2) = 0.693 as a guide: If a culture has a specific growth rate ~ 0.70 day-1, then its cell population is doubling once per day.
Computing this KPI for seed fermentation and then control charting this KPI is the best start you can make towards monitoring and controlling your process variability.