How much do I offer an investor in multifamily project

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Hello. I am a new investor, building a $4.8 million rental townhouse development. I had two parcels and approached the owner of a parcel that split the two and asked if I could buy it. He said no sale but that he would like to be an investor in my project. I own the main parcels (80% of the land with the utilities--appraised at $450k); he owns the other 20% ($116k), which is a split off a multifamily property he has. Of the $950,000 in downpayment, he is giving $300,000, along with his land and signing on the loan (I probably couldn't get the loan without him or at least it would be tough). The project has been appraised before built and out of the chute will be worth $6.2 million. The NOI is $466,000 for years 1-5; $404,000 in years 6 forward (after tax abatement runs out). Best term sheet from bank puts cash flow at about $190,000 annually for first five years. We may hold the property as rentals for a long time, or split and sell as condos down the road. I played around with some of the ways of looking at value in the Introduction to Real Estate Analysis article by J Scott, but whenever I think of an offer I get worried I am either insulting him or giving away all my hard work (3 years of time assembling lots and getting it through city approvals, and serving as an uncompensated GC for net zero energy certification and most of the site work). Does anyone have suggestions for how I approach this (knowing, of course, he can ask for what he wants). Thanks for your input! --Cati