I'm sure if you were to ask most investors at which point in the investment property lifecycle they break-even and truly start cash-flowing they either haven't figured that out or don't know how to calculate that number or what the true IRR on a property is going to be because they think once they close escrow with 20% down they are cash-flowing on day 1.When @Chris Seveney talks about the IRR for a long term buy-and-hold, I factor in a rehab at year 15 of a 30 year hold, whether it's due to wear and tear, or possible change in interior styles to remain competitive in the market.

You (generally) are trying to factor in “personal enjoyment” or “personal use” as your upside.

In the book "What Every Investor Needs to Know About Cash Flow" the author talks about not just vacancy but also factoring in rent that you can't collect for various reasons, for example a tenant who refuses to pay but is still living in the property (not technically a vacancy).

Assuming that assigning is not a desired option, what are the determining factors in choosing to use transactional funding vs using C Buyers funds?

Just factor it as a cost of doing businessWhile California deems the LLC is doing business in California doesn't necessarily mean that the fund will have California state sourced income.If the LLC files a partnership return and all the payroll, property and sales are outside California - The LLC will not have any California state sourced income.

I'm a landscape designer for a landscape architecture and planning firm in Costa Mesa, California.

@Matt Gregory the main element I would ask you to think about here is that a HELOC is designed as a temporary loan. 

I'm not sure how to design this business partnership. 

Answer will largely depend on;A) Language of the lease agreement,B) The local laws related to enforceability of language in lease agreements.No answer can be given without knowing both factors, as well as knowing how your local judges would rule on same.

@Hadar Orkibi, Hi,  You need to factor in the Seasoning of 6 months into your calculations.