Here is a good site if you need to get a good idea of how much gain a certain size dish will have. I have found it very useful for making comparisons to different size dishes. The site link is posted below. EB

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The calculation results are:
* The total side to side beamwidth of the antenna main beam.
* Half the side to side beamwidth, if you are concerned to determine the offset angle from the beam centre.
* The on-axis gain, relative to an isotropic omni-directional antenna.