I posted earlier down that iota was 9, but you are right - in counting it is 10 even though it is the ninth letter of the alphabet. Now here is the solution to the digamma/"stigma" (I think you meant sigma) problem you have:

Basically, the most common ancient Greek dialects which have come down to us are Athenian and Attic, which most people study. In those dialects, the final sigma, which looks just like an "s", is used for 6. In the less common and zanier Doric dialect, the digamma was probably used. The greek alphabet does not have enough characters in its standard form to represent a base 10 number system well, so such letters were probably placed for this reason. In either case, Chi (usually sounded as "ch" but often "c") and iota are as you said previously.

Good work!

> It's pretty confusing, but i've found more information supporting the use
> of [ which equals STIGMA = 6

> The Stigma [ is an exact match to the symbol found here.
> http://www.parthia.com/font_stigma.htm
> Apparently it could be found on greek or romain coins.

> do we add them together or us there alpha values which are S,I or their
> numerical value 6,10 or 16, 610.