Roman legionaries would throw pila before charging when enemies were close - perhaps as close as 15m.
There is no way to replicate this in the TW engine that I know, so the best practice in my opinion is to seek to emulate the results rather than the actual number of meters. A unit should be able to shoot off a volley of javelins into another charging infantry and still have time to initiate their own charge.
If you wanted to actually re-enact an ancient battle in a TW engine it would be extremely difficult. For instance, it appears that close combat was fought in short bursts followed by disengagement and re-engagement rather than a sustained fight that took hours. The TW engine was built to simulate cinematic combat, not that, and the best you can hope for, I think, is to get believable results (low lethality of engagement, heavy use of skirmishing) rather than getting all the details correct.
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