Not sure what Form 540 is. What should I enter for "Use Tax" line 91? This section came up under my State Tax Review

[MollsBBU]

Form 540 is the state of California Income Tax Form.

You are required to enter a number on this line. If the amount due is zero, you must check the applicable box to indicate that you either owe no use tax, or you paid your use tax obligation directly to the California Department of Tax and Fee Administration (formerly known as the Board of Equalization). 

You may owe use tax if you make purchases from out-of-state retailers (for example, purchases made by telephone, online, by mail, or in person) where California sales or use tax was not paid and you use those items in California. 

If you have questions about whether a purchase is taxable, go to the California Department of Tax and Fee Administration’s website at cdtfa.ca.gov, or call its Customer Service Center at 1.800.400.7115 or (TTY) 711 (for hearing and speech disabilities). 

- https://www.ftb.ca.gov/forms/2017/17_540bk.pdf - pg 15

[devouy]

How do I know what number to enter?

[BillM223]

See this CA FTB page to help you determine the amount of Use Tax to enter.

@devouy 

[michael_s_peterson]

If all of your purchases come from within California, enter 0.  And you will have to check the box stating that No Use Tax is owed.