You should use Form 2, Schedule III, to calculate your itemized deductions, but remember that your Montana itemized deductions may be different from your federal itemized deductions. There are federal deductions that are not allowed on your Montana tax return and state deductions that are allowed only on the Montana tax return.
If you are married and you are filing separately on the same form or on separate forms, you should apply the following rules:
● Both spouses should either claim the standard deduction or itemize their deductions. You are not allowed to claim one method for one spouse and another method for the other spouse.
● Deductions that are attributable to only one spouse have to be claimed by that spouse. This includes, for example, federal income tax withheld from your wages and your employee business expenses reported on federal Form 2106.
● Payments made from accounts owned by both spouses and that pertain to both spouses can be allocated to either spouse in any proportional amount.
http://revenue.mt.gov/Portals/9/individuals/forms/Form_2_booklet_2014.pdf