Best compression level when compression iso

What is the best recommended compression level to uses when compression your iso to cso.

[Kanadier]

It all depends what size the ISO is. A 400 mb one would be best compressed to 200 mb

on the compressor program, it has a compressor level. the levels are from 0 to 9. Can I uses a lower compressor level or the level has to stay on 9.

[brentbizzle]

9 is the most compression, but i don't know why you would want to change it from there. I guess if you experience some problems with a cso... I have yet to have any issues.