The IRS often uses the word "calculation" as a very generic word---so not sure what your issue really is.
Phone support is not provided with the Free Edition. If you are using a paid version of the software or if you purchased PLUS you can get phone support when customer support is there. Otherwise, post your question here and someone will try to help.
To call TurboTax customer support
https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number
Customer support is available from 5 a.m. to 5 p.m. Pacific time Monday-Friday
TurboTax provides do-it-yourself tax software. TurboTax guarantees the calculations done by its software. The information entered into that software is data entered by you, the user. TurboTax cannot and does not check that information against any outside sources, so TT cannot know if you entered your information completely or correctly. It is up to you to enter all of your information correctly and completely, and to check your tax return before you file it. If you forget to enter some income, or enter it incorrectly, enter it in the wrong place, etc. then you may end up owing additional tax to the IRS and/or state. TurboTax does not pay the additional tax you owe. If TurboTax’s calculations were incorrect, and you owe penalties and/or interest, then TT will pay the penalties and interest. The tax due is always owed by you, and must be paid by you.
If you need to enter a claim for the accuracy guarantee: