I have a duplex in a quiet neighborhood and have a prospective tenant who sounds very responsible, has good credit and income.  

We have excellent credit and no debt except for our 2 mortgages.

*Provide information about rental rates and other terms and conditions of the rental, as set out in a schedule provided by the owner, and*Accept deposits, fees for credit checks, and other administrative duties, security deposits, and rent*Provide preprinted rental applications and respond to applicants’ inquiries about the application*Show rental units to prospective tenantsAs you can see, a non-licensed, non-resident manager is limited to performing rather routine tasks that don’t call for initiative or decision making.For instance, this manager would not be qualified to negotiate a lease (4), deal with late rents or other violations of the rental agreement or lease (5), or appear in court on your behalf (6).(1) My property manager does NOT live on any of the owners properties.(2) My property manager is unlicensed.(3) My property manager supervises 10 different properties.(4) My property manager has negotiated leases for 12 years without a license.(5) My property manager has dealt with late rents and violations.(6) Unknown

Generally, this means commercial properties with very good credit-worthy tenants signed on a very long term lease. 

What is a fair/typical % to apply as credit toward purchase price?

When you partner with someone, the money they put into the property isn’t counted against you as debt on your credit score which can have a negative effect on your ability to get loans etc. that is one of the benefits of partnering with others.

The gap is usually due to a few % taken by their affiliate company for the credit card clearance.

My credit score is 830 and I have NO other debts - just my current mortgage.

In terms on a HELOC or a Line of Credit, what's the best starting point for looking in to this?

If your asset has appreciated well enough, you can take a line of credit or perform a cash out refinance.