How best to calculate mortgage interest and taxes for business use of home when the business is active for only 3 months and the balance of these deductions carries to Sched A?

[qbteachmt]

As long as it meets the "dedicated space" requirements for a home office, you would use the info from the IRS:

https://www.irs.gov/publications/p587

Search on the word "seasonal."

[dkh]

Form 8829, Part II Allowable Deduction has a Smart worksheet for Line 8 to enter date began use of  home office and allocation of Sched C income.   That should help limit your deduction.