Chain length depends on use. A racer wants the shortest possible chain length because it makes for faster shifts. The tourist wants the longest possible chain because in the event of a broken link, he can remove it and continue with his journey.

The method described above (big/big + 2 links) gives you the shortest chain length that should not break the RD.

To find the longest possible chain that will work: wrap the chain through the RD and the small cog and small chainwheel. Find the longest chain length that will still cause the RD to wrap some chain. You're now guaranteed that the RD will always take up chain slack.

The derailleurs will function with any chain between the minimum and maximum lengths. Your choice depends on how many times you anticipate breaking the chain and how inconvenient it might be to replace a broken link if you do break one.