Mines need to be in the same square as the attacking ship. If you want to defend a warp point, then it's probably best to put the mines right on the warp point.
I assume you're talking about the normal systm view, not the combat view.
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SE4 Code:
L GdY $ Fr- C- Sd T!+ Sf-- Tcp-- A% M>M+ MpD! RV Pw Fq+ Nd- Rp+ G-
/SE4 Code
Work out your code at
http://www.sandman43.fsnet.co.uk/se4main.htm