When buying a house using 100% private investors money, what are some tips on fair ROI for the investors? Also if the house is going to be flipped, what is a reasonable estimation of profit after capital gains tax and investors ROI is taken out. I may be running my number wrong because it seems not worth the time to do this.

You are likely running your numbers correctly.  Not every home is a candidate to be flipped and there could very well be a fair amount of competition in your area where more lucrative deals are getting scooped up quickly and others are cultivated from off-market properties.

As for fair ROI, you'll likely hear some typical CoC figures but that's something that could be different for every investor. Depending on who you are working with, some may be happy with anything higher than a savings account while other will want higher double digits. The deal itself can make a difference as well based on the acquisition price of the property, rehab, timeline to complete and ARV, as it isn't always paint by numbers (i.e. the 70% rule).

Calculating the gains may also be something that's better suited for a CPA in your area, because there are a number of factors that could influence the outcome.

Sorry for being vague about your inquiry, but unfortunately like many things within the forums the response is often, "it depends".  I would first determine how you value your time, as it seems that based on your calculations it's not worth it already.  If your heart isn't in it, I'd explore other avenues that may provide the return you want in an area you find more interesting.

"I may be running my number wrong because it seems not worth the time to do this." - Maybe real estate isn't for you. Nothing wrong with that assumption. If you have a great deal finding money isn't that difficult. 

The ROI will vary depending on the source.