The last I looked it up, I concluded the right answer was to bifurcate the property into 2 Sch E entries, one for the self-rental and one for the remainder. It's not all or nothing based on percentages, the way residential vs non-residential is.

That said, if the "his business" is not something that files a separate tax return (that is, if the business is a Sch C), and if "his LLC" also does not file a separate tax return (that is, the rental goes straight on Schedule E, rather than Ep2), actual expense for 50% of the building goes directly on Sch C, the 50% rented to other tenants is normal rental income and expense on Sch E, and the self-rental income / expense is entirely eliminated.

Client business is an S-Corp and the rental is reported on Sch E (Single member LLC) for clarification.

For my client with a similar situation, including entity type, I have one Sch E column for the self-rental and one for the remainder.

Did you allocate the expenses based on the allocation of self-rental income to total rental income for that property?

The only client I have in that situation currently gives me square footage, so I allocate based on that. Any reasonable method is reasonable. 

Here is an article that references the final QBI regs. You should read through both the article and the regs to answer your question about the self rental and SSTBs. https://www.withum.com/resources/do-self-rentals-qualify-for-the-20-qbi-deduction/

Great article...........thank you.