The "70% Rule" is very common for flips. The majority of seasoned investors will use to gauge whether a property is worth looking into any further. I find this appropriate for flips, or at least close. I recently did a deal that was actually about 74% resulting in a fairly small profit, comparably, but it was only about a 3 week rehab so our hourly rate was well above our average. Just depends on your tolerance for risk, time frame for turn around and financing.
As far as rehab and hold, that gets tricky. Buying a property that needs renovation is a great way to maximize cash flow and reduce maintenance, but it can be difficult to analyze because you still have to consider your downtime. I like to look at all of the same factors as a flip and add up what my total expenses will be to go from closing to placing a tenant, and also add in lost rent money during renovation(3 month reno x $1000/mo in rent= $3000 in additional cost). Take that total and compare it to what kind of property is available rent ready.
As an example, let's say you find a 2 bed 1 bath with a garage that should rent for $1000/mo that's listed for 60k and needs 20k to get rent ready with a 3 month reno and you're paying all cash. 60k purchase+20k reno+6k holding costs(3k in lost rent, insurance, taxes, utilities)+whatever your time is worth= at least 86k. I factor in insurance and taxes since you'll be paying those out of your funds until the unit is rented and then you should be budgeting them out of the rent monthly.
Take that 86k and look at what you could buy with that. If you're coming out with other properties that are comparable, I would buy one that is rent ready or keep looking. If your finished project is more comparable to a house that would cost closer to say 105k, then it's probably a decent deal. Obviously there are other factors to consider, but its easy to look at a "cheap" house that should rent for what seems like a lot compared to the price but over look the length and cost of the renovations necessary to get it rent ready.
The other consideration here is how you're going to finance your renovations. I will pay for flip materials with a credit card because I know I'll only pay interest for maybe 2 months and pay it off when I close on the property when I get the "big" check. However, I only pay cash for renovations for a property that I'm going to rent because there is no big pay day. My credit card only carries 9.2% APR but if I'm paying a hundred-ensome dollars in interest a month and hoping for $2-250/mo cash flow from rent, I'm eating into that hard until that credit card is paid off.
I hope that's helpful and makes sense.