I have my 2 adult children under age 26 on my insurance through the health insurance marketplace, how do I figure out percentages

[RobertB4444]

If you are splitting your marketplace coverage with your adult children who are filing their own returns then you answer yes to the question "Are you splitting this coverage with someone else and then you divide the numbers equally among the number of people being covered. 

So if it was just you and your two children then you would divide by 3 and everyone enters one third of the each of the numbers on your 1095-A into the tax return. 

If it is you and your spouse splitting with your two adult children then you divide by 4 and each of your children gets 25% while you and your spouse would put 50% of the amount on your return and so on.

[Mike9241]

The tax laws allow you to split the amounts any way you want. it does not have to be 1/3, 1/3 1/3 unless your children disagree.  

Under the rules in this section, you and the other
taxpayer may agree on any allocation of the policy
amounts between the two of you. You may use the
percentage you agreed on for every month for which this
allocation rule applies, or you may agree on different
percentages for different months. However, you must use
the same allocation percentage for all policy amounts
(enrollment premiums, applicable SLCSP premiums, and
APTC) in a month. If you cannot agree on an allocation
percentage, each taxpayer’s allocation percentage is
equal to the number of individuals enrolled by one
taxpayer who are included in the tax family of the other
taxpayer for the tax year divided by the total number of
individuals enrolled in the same policy as the individual(s).
The allocation percentage you use and that you put on
line 30 of Form 8962 is the percentage of the policy
amounts for the coverage that you will use to compute
your PTC and reconcile APTC