Speaking generally, since I do not know all of the specifics of your case, I will answer your headline question. How easy is it to impute minimum wage to a custodial parent? Very easy. Under the Family code, parents are required to be self sufficient. It is also a requirement that parents provide for their children, and that means working. A parent that is still of working age cannot unilaterally refuse to seek employment without having at least minimum wage imputed to that parent at a support hearing, especially if there is recent work history. This is true even if that parent has returned to school and has most of the custody time. If you want wages above minimum wage imputed however, you will need evidence that the person can earn income that is above the minimum wage, and that requires a showing under the family law code that is far more arduous.
Without knowing more details about your case, it is impossible to predict how such a request would be ruled upon by a judge or predict how much it would cost to hire a vocational expert, or whether that would even be necessary. You should not attempt to do this yourself. Making a request for the imputation of income requires evidence and a well drafted declaration that would justify the court imputing earning capacity rather than actual earnings to the custodial parent.
Each case is different, and what you ask is very fact dependent, so my answer will be more general. Nevertheless, I suggest that you consider asking for a Seek Work Order, since both parents have a duty to provide for the needs of their children, and refusing to look for work will not impress the Court. Until recently in Santa Clara County imputing minimum wage when a parent did not diligently look for work seemed almost automatic.
Imputing your ex's former teaching income using only her past paychecks will not work. You need to show both ability and an opportunity. If she files for support you should speak with a good attorney about your options.
As stated above -- I have NOT actually stated any legal opinion, because I need to know more before I can start to form one.
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