First, welcome to the community. I'm not sure I understand exactly what you want and from which platform you are using to create your mixture design, but you can certainly do linear constraints and set the value after the ≤,≥ sign in the DOE>Classical>Mixture Design platform:
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You can also do this in the Custom Design platform after you have added your mixture factors.
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"All models are wrong, some are useful" G.E.P. Box