SE health insurance deduction

What was your coverage before you started your business?  You can't take the SE health insurance deduction for any month in which you or your spouse were eligible to participate in an employer-subsidized health insurance plan.  Eligibility for the SE health insurance deduction is determined on a month-to-month basis.

Also, your SE health insurance deduction cannot exceed the net income earned by your self-employed business.

Then........If your health ins exceeds your net self-employment income it gets split. An amount equal to your net self-employment income goes on Form 1040 Schedule 1 line 17 (to 1040 line 10) and the remainder gets added in to medical expenses on Schedule A.

Another way to explain this is that you can only use the small business deduction for an insurance policy you buy in your own name or in the name of the business.  An employer-sponsored policy is not eligible even if you paid 100% of the premium.  If you start your business on July 1 (for example) I doubt that you would buy a policy that was backdated to January 1.  

On the other hand, "registering" a business with your county or state is not necessarily when the business "started".   Your business starts when it becomes an ongoing trade or business activity for the purpose of making a profit, and you are required to report all your business income and expenses, even if the business is not registered with some government agency.  For example, if you want to do landscaping and lawn mowing, and you start canvassing neighborhoods and posting ads in April, that is when the business started, even if you did not get your first paying customer until May or did not get a local business license until July.