You can access the "Calculate Flux Limits" option under the "Analysis -> Flux Variability Analysis" menu or right clicking on the Project/Metabolic Model icon on the clipboard.
This option allows you to calculate the minimum and maximum possible values for all the fluxes in the model. The simulations are always performed using Flux Balance Analysis (FBA) by maximizing and minimizing each individual flux.
Define the minimum value for the biomass (in %).
2.Select Environmental Conditions
If you have created environmental conditions you can select them to be use as additional constraints.
Press OK, and check the result in the clipboard.