Yes, you need to come up with the actual budget that you can tell your kid before the college application list is made. Yes, it may require some hard talks with your ex… but (as mentioned above) you do not want to tell your kid in April that all of the admissions are too expensive.
Only after the actual budget is set can you use net price calculator results to get an idea of which colleges may be affordable. Of course, remember to check each college to see if the parent income for that college is $130k (custodial parent only) or $280k (both parents) before using the net price calculator.
But also note that for colleges using custodial parent only, any money the kid receives from the non-custodial parent (e.g. for college costs) will be added somewhere, and used in financial aid calculations.