Post Fix
The amount of post fix required would depend on the size of the hole the post is going into.
When using postfix, position post vertically in hole, open the bag and pour half the contents of the bag into the hole around the post. Using a watering can fitted with a fine rose, pour clean water onto the base of the post at the level of the post fix. After 2 minutes, check the vertical position of the post with a spirit level (front and side faces). Adjust as necessary. Pour the remainder of the post fix into the hole and add more clean water, directed onto the post at ground level. Allow 15 - 20 minutes for initial set to take place, prior to commencing with further work.
The posts should be buried approximately 600mm into the ground and the postfix is to be placed around the post, but not underneath. If a post has the concrete underneath as well as around the sides the concrete can trap water and therefore soak the post greatly increasing the chance of decay.
If the ground the post is sunk into is not free draining soil then a small layer of Shingle can be placed under the fence post, (with the post still sunk a minimum 600mm into the ground,) to help drainage occur.