What form for in-plan 401(k) Roth conversion "income"?

In December 2018 I made an in-plan conversion of funds within my 401(k) plan,  from original non-Roth to Roth.  Just as with a Traditional to Roth IRA conversion, I know the money converted will be taxed as ordinary income.

But, I can't figure out where to report it in TurboTax.  I can't even find a web reference stating which IRS form to use.  It's most likely Form 8606, Lines 16 and 18.  But all the text there is specific to IRAs, with no mention of 401(k)s, which concerns me.

Form 8606 takes me to creating a Form 1099-R, since the financial institution hasn't sent one.  There I entered the $$ in Box 1 Gross Distribution and in Box 2a as the Taxable Amount.  In Box 7 I selected '7' for the 1st code (normal IRA distribution), and left the 2nd code blank.  This resulted in Box B4 showing the conversion $$ amount, and I checked the box under that.

On Form 1040, the conversion $$ amount appeared in Line 4b as I think it should.

Does anyone actually know - as opposed to guessing like me - the correct place / codes etc?