@Seyed Javaheriwelcome to BP. I'm not sure I understand the question so I will do my best to answer each part.
1. I need to be convinced that 70% formula works in most cases.
You should not want to be convinced. The 70% ARV (less repairs) is a guideline and it is a far cry from a "formula". It is there to help investors when other factors are not present. Happens to be it works many times, however, if you look at the extremes it will likely dispel any myths about it being formulaic. For example, if you are selling a house with an ARV of $10M, you will probably do okay if you go above 70%; and on the flip side, if you are selling a house with an ARV of $25K, I would probably advise that you don't want a razor thin 30% (or $7.5K) to pay off all of your other expenses (outside of fixing the home) and feel good about walking away with a profit.
With that said, in our market of homes from $300K - $600K we will gladly buy 70% ARV (less repairs), we will even buy homes closer to 75% ARV (less repairs).
2. In my humble mind, how can you convince the seller to accept the offer since the listing price drop will be substantial.
Most of the time the seller will not sell at a steep discounted price. However, sometimes the seller needs help and if you can help solve their problems the price is not the only consideration they are thinking of.
3. What is the maximum acceptable number?
I assume you are asking about the maximum offer you can make to a seller. [Assuming I understand your question correctly,] the max offer you should ever make to a seller for an investment property is the amount that if the seller were to accept your offer you can hit your minimum return you were looking to make. For example, if you would be happy with no less than a $5K return, then your max offer should reflect that you will not make less than $5K after completing the deal.
Hope this helps clarify some of your questions. Best of luck!