Those are added to all the other medical expenses on the Sch A if you itemize deductions. 

No I don't think that is correct. I am not itemizing my deductions. I am claiming the self-employed health insurance deduction which shows up on Line 29 on the new 1040 Schedule 1. What shows up is the data from entering form 1095-A but I did not get one of these for separate dental insurance premiums.

First, if you got the insurance thru the same company thru the ACA then both premiums will be on the ONE 1095-A form they issue.    If they did not you will enter the dental insurance in the Sch C input section as if you did not get the 1095-A.... this is most unusual.  

No. I did not get insurance through the same company on the ACA. As non-pediatric dental insurance does not get a tax credit there is no 1095-A issued. But I understand it is still included in the self-employed health insurance deduction.

Also I am talking about an S-corp with a K-1 so there is no Sch C associated with this self-employment, so I don't understand how I can input it into this section?

I also can't work out how to manually override the number in Turbotax desktop.

First NEVER override anything unless specifically told to do so by TT support ... overrides void the accuracy guarantee and may keep you from efiling.

Next, mentioning the S-corp to begin with would have been helpful ... watch for the screen in the K-1 entry section that asks about health insurance for you as an employee since you are an employee and should have been taking a salary and issuing yourself a W-2  since you are technically  not self employed.  @dmertz can you assist ? 

Thank you. However, the instructions by that check box do not make sense to me as I did not personally pay health and dental insurance premiums. The S-corp paid them directly. The amount is included on my W-2 as wages.

To the other poster, yes, I do have a reasonable salary from the S-corp and a W-2. That is not the reason for this post.

Thank you.

However, following your instructions does allow me to add this extra premium for dental insurance paid directly by the S-corp and included in my W-2 wages, and calculates the deduction correctly. Not an entirely clear paragraph from Turbotax IMHO. Thank you for your kind assistance!

Thank you. This is a very clear answer.

I appreciate the explanation by dmertz. It matches what other experts have posted on this question. However, it produced a faulty result in my case. (I’m using TT IRS Free File.)

First I entered the data from Form 1095-A. TT filled in “self-employed health insurance deduction” (Sch. 1, line 16) with the correct number: my out-of-pocket payments for medical insurance minus the PTC. So far, so good.

Then I entered my dental insurance payments in the K-1 section as dmertz describes. This caused Sch. 1, line 16, to drop by a large, random amount — the opposite of what it should have done.

TurboTax doesn't seem to always produce an optimal result when the iterative calculation does not converge.  It might be necessary to do a manual calculation and explicitly enter the self-employed health insurance deduction amount.  See the Self-Employed Health Insurance Deduction and PTC section IRS Pub 974:

https://www.irs.gov/pub/irs-pdf/p974.pdf