Total parallel connections between 10 and 30 should work good.. depending on your system CPU and memory...
How much you spend on a single file depends on how much priority you give to a single file. The higher the downloads per file is the lower is the chance that you download more then one file in parallel.
I usually use half of the total.

The segment size and target time depends on your usuall download size and waiting tolerance. A segment size from 16 to 256 is usually good. After that the target time comes into play.. I dont mind waiting a while for the last segments. Thats why I have this value usually at 5 minutes. You should not go below 16Kb and 60 sec. though... It will raise the transfer overhead...

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