I first heard the 70% rule (really more of a guide) from Ron LeGrand back in the late 90's when I started investing. The reason you do it is because that 30% buffer covers most of your costs such as Purchase Costs (3% budget to cover initial acquisition cost, brokerage fees & closing cost), Holding Costs (3% budget to cover monthly property expenditures while you're doing rehab), Sales Costs (6% commission when selling) , Miscellaneous Cost (3% unanticipated expenses) and of course, your Profit (15% or more if you have any left over from your cost budget.
Now, if you do it in reverse, on your $100k example don't look at it as a $6k differential but rather 6% based on the ARV. When you start rehabbing higher priced properties, like where I am in Northern NJ/NYC, that 6% could be very significant.
I only use this napkin calculation in the very beginning on my first conversation over the phone with the seller to determine if it's worth my time and go see the property (first filter so to speak). If and when I do visit the property then I break down all my cost.
Hope this sheds some light on your question. Good luck.