Everyone's investing journey is different based on their own situation and looking at all the factors like time, energy, life situation, and the raw numbers will guide your decision on the best investment for you. 

Raw land alone is selling at 100k per acre in his area.

I think if I can find out where to get listings off market as you suggested, my underwriting would be more fruitful for sure.

However, these properties won't be low hanging fruit.

At the end of the day they're the ones that get the lowest hanging fruit. 

What should be the value of raw land (in percentage) from a given price of a built house ($750K)?

If you at least have a decent amount time, then you can try to find low hanging fruit on FB Marketplace, Craigslist, cold calling land owners. 

I have toyed around with the idea of buying raw/vacant land, and running the site planning thru my firm, then selling that shovel ready land for a profit.

Andrew, you bring up some very real and raw issues regarding REI that many don't talk about. 

Assuming you are just now beginning investing, I recommend picking the 'lowest hanging fruit' and 'walking path of least resistance.'