NavList:

"Eratos The Knees" will work very well.  In terms of the syllable with the most emphasis, you can either let yourself be guided by English-language rules, which would tend to put the emphasis on the "tos" syllable.  So eraTOSthenes.  And the final "es" syllable would sound - just as you nave it - like the word "knees".  This is what I would do personally.

Or you could try to echo ancient Greek a bit more (which goes against the grain of my English-Canadian brain) and put the emphasis on the "the"...as in eratosTHEnes...where the vowel sound of the final "e" is like the "ey" in "they".

The "sth" sound in the middle is the same one we use in the word "esthetics"...but you already have that.  And don't worry about needing to say it fast.

Finally, you don't need to obsess about the proper way to say this.  There is a bit of ongoing debate as to exactly how ancient Greek was pronounced, and European scholars use a slightly different pronunciation scheme than North Americans do. 

The take away for you is that what you are doing is absolutely fine...and there is a fair bit of latitude in how you might pronounce this.  The key is to be bold, look people in the eye, and don't ever apologize with "I'm not sure how to say this, but...."  Just say the name and move on.

Bob Goethe