Couldn't see how old the post was, but normally thumb and froefinger should be used.

I guess experienced users might also be able to use the middle finger.


About the reverse order discussion: technically there is no difference, but I suspect there is a speed advantage in reversing the order, and maybe also helps in the mechanisation.

The pros say:

When adding, always add first, then apply the carry.
When subtracting, borrow first, then subtract.

By "using his brain" he might be doing mental calculations. Normally, the idea is to perform the operations in a mechanical way, which means he is obliged to do everything on the soroban. That's the point of using a soroban.