Let me start by saying I'm also a newbie.
From what I understand, it gets better with experience. I made my first offer yesterday and spent so much time trying to have the most accurate figures as to not be one of those wholesalers who are way off on their numbers. But when i look at other contracts wholesalers put up in my area, most of them just say minor, moderate, or major rehab on the repair costs. Really, diff people can get it done for diff amounts depending on their own experience. I think a fair place to start is for a ~1500 sq ft home, light or minor rehab(paint and carpet, cosmetics)10k, moderate or average(add to that kitchen and bathroom update)25k, and major or heavy(everything redone)50k. Again, these are just starting points. I find studying what other wholesalers put up on facebook and other media in my area really helps. You can see which contracts are getting attention and the compare their pics to their est rehab costs.
Also "The book on estimating rehab costs" is constantly recommended to me