Well, my technique might not work for you. In my situation, my ex was in kind of a "hobby business" for several years before we split. My position was that she could have ended her business partnership and gotten a regular job in the same field (she's very qualified and I'm pretty sure there are openings in the field).
She decided that she wanted to keep her business as a part time endeavor and get certified to teach as a full-time career. My lawyer told me I wasn't necessarily obligated to support that (with spousal support) so I believe I could have gone the vocational exam route, gotten her imputed with income and gotten support reduced. That would have cost me a chunk of money for the evaluation and court fight, but I'd have probably gotten it back in reduced support within a year or so.
But since she was actively pursuing her teaching credential and will likely be making about as much as a teacher as she would in her other field, I made an offer that I would continue to pay the higher support while she was getting certified in return for a larger share of cash and stock in the property settlement.
So I kind of feel like that worked out as a win-win for us, but it might not apply to your situation if your ex doesn't genuinely intend to do what she can to support herself.
Maybe a first step for you, if your ex is uncooperative, would be to file a request for order that a Gavron warning be issued to your ex officially reminding her of her responsibility to become self supporting within a reasonable period of time. Possibly that warning from the court will concentrate her mind and get her going. Either that or go ahead and get the court order for the vocational exam, but then tell your ex you won't push it if she'll discuss voluntary reduction in support.
But again, the wildcard here is how much of her time she needs to devote to child care--number and age of kids would likely enter into an evaluation of her ability to work even if she's highly qualified.
Also, I'd definitely recommend running all this by a qualified family law attorney--they can look at your specific situation and give you a better idea what's feasible for you.