There's a whole procedure outlined in the service manual. Screw location depends on whether you have xenons or not. Basically you'll need the specs from the manual, a flat wall, tape and a yardstick in millimeters.
Here's a manual for sale on ebay--you might ask the vendor if it includes the aiming procedure.
http://cgi.ebay.com/ebaymotors/Auto-Repair-Service-Manual-Subaru-Legacy-1990-19
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Z180185597302QQrdZ1QQsspagenameZWD1V
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