It's called "imputed income" and I believe you can use this in Missouri. You'll need evidence of her capacity to earn money, this usually takes the form of an examination by a qualified vocational evaluator. You'll probably need a court order to require her to participate in the evaluation. The evaluator should look at both ability (does she have skills and availability) and opportunity (are there jobs available.)
What my lawyer told me (California) is that even someone with no work skills or experience will typically be imputed with minimum wage, and for someone with some work experience and education even if it's old, something more like $1500/mo is typical. If she's got specific skills and experience as indicated by the vocational examination, imputed income will probably be whatever is typical for those kinds of positions in your area.
If you have kids and she's the custodial parent you may have a hard time demonstrating that her time is better spent working rather than taking care of the kids, but if there are no young children and she's of working age, has some work skills and is healthy you've probably got a good shot.