On what basis does the 'right' in Buddhism come from?

There is no 'right' and 'wrong' in buddhism. There are two word. 'Samma' and 'kushala'. Both are trying to help in discriminating based on the goal i.e Nibbana. Kushala means skilful that helps you towards your goal. Here your goal is nibbana.

Similarly Samma has various meaning based on context. But main theme is BALANCE. In case where optimum is important. I.e not "too much and not too less" then Samna word is used. But still the purpose is to stay put on the path(Nibbana).

When I read this question I felt this is like someone catching the snake from its tail. If we catch the snake from the tail it can bite us. but if we catch the snake from the head or neck it cannot move or bite anybody. Likewise we need to catch the dhamma in a right place. Without knowing the theory correctly if we catch the word and start experimenting with the knowledge of our max 50 to 60 years[our age] it is not fair when we compare with the our Lord buddha's knowledge. We have to trust and stand the Point lord Buddha is always right. I might get wrong somewhere and I need to find where I got wrong.

If that is the case you can find the answer easily. In your case if you study the "Patichchasamuthpada" the causes and the effects. You will find the exact answer for your question. Also, you need to study how the "Karma" works on each mind. Then you need to learn Chiththa -01[89 or 121 in actions], and chaithisika -52, and Chiththa Vithi[Who the 17 minds together completing the action]. Once you learn these you can find where exactly you get the "Samma Right" or Wrong.

When the consciousness is in action each mind working with 52 chithisikas. Mainly it has 25 Beautiful or wholesome factors and 14 unwholesome factors which occurs due to 6 causes. Greed[Loba], Dosa[Hatred], Moha[delusion]which creates unwholesome mental factors and Non-Greed[Aloba], Non-hatred[adosa]Wisdom[Amoha]which creates wholesome factors. 7 Universal mental factors are there in all minds irrespective of the wholesome unwholesome acts. 6 Occasional factors vary change accordingly to the wholesome and unwholesome minds.

This is a very small brief that I wanted to prove you exactly how you can find the line of the Right[Samma] and Wrong[mithya]. To Learn exactly you need to learn Chiththa, chaithicika, rupa, Nibbana. Lord Buddha's speeches he never utter any single word that may not helpful to the society. All his words directly aim to the Nibban. If we can direct our actions and thoughts to the nibbana it is not too far. It is already within us. But Nibbana is surrounded by lot of garbage in our samsara collection. We need to meditate and bring this up by removing all the garbage surrounded Nibbana then it starts blooming. May Triple Gem blessings helps you to find Nibbana Soon. If you willing to learn just follow the link and follow lessons.Loving and Kindness

The basis of the right is within the gambit of the soteriological context and causation.

The soteriological context is removing suffering (Nirvana) and maximizing happiness (Buddhahood). The causation involved demonstrates the principle of causation (dependent origination) and its comprehensive account, with regards to mental states, phenomenon in general and the soteriological context.

The right is right on the basis of the causation. As an example, a hired assassin also needs immense amount of concentration (amongst other skills) to carry out his task. But this concentration is not beneficial for attaining Nirvana or happiness. And thus, such concentration is wrong concentration. Such, is the basis of the ''right'', in short.

Think of it this way... If you want to go to a friend's house, there are probably dozens of paths you can take to get there. Some are shorter, some are more pleasant to walk, some go by your favorite coffeeshop or bookstore, etc. There are also many, many, many paths you can walk that will take you someplace else entirely, so that you never reach your friend's house at all. The 'right' path to your friend's house is whatever path gets you there reasonably quickly, reasonably pleasantly, and reasonably safely.

That's all that 'right' means in this Buddhist context. The eightfold path spells out things that monastics should adhere to so that the journey toward attainment will be fast and direct. It's a roadmap that keeps one out of the mire and free of unforeseen obstacles.