How far apart do I set my posts for a vertical Slipfence system

Post should be set to 93" between each post.

DO NOT set post wide than 95" between posts because the stringers are manufactured at 92.5" and the brackets have an oval opening where it is attached to the stringer via a carriage bolt of approximately 1.5" on both sides to provide up to 3" tops for adjustments 

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What if I set my posts further than 8' centre to centre

As long as the posts are within 95" between the posts then the panel will be fine to assemble. If the posts are set further than 95" between posts then one or more posts may need to be reset in concrete.

What if I set my posts shorter than

This is not a problem, the stringers can be cut to any length with a metal blade on your power saw or hacksaw. 

How can I cut the stringers

The stringers can be cut as a pair with a metal blade either on your circular saw, mitre saw/chop-saw, or reciprocating saw, (difficult to cut straight), or a regular hand hacksaw..

First make sure when you have the stringers beside each other that they are the exact same way with the same hole patterns, then sandwich them together, place one strip of masking tape around both of them so they stick together when cutting and then cut slowly both together.