Can anyone tell me what my water pressure should be set at ?

I have a Micron 50 FF boiler, this is in the kitchen about 4 feet off the ground and I have a Flexcon pressure gauge in our airing cupboard (ground level) about 10 feet from the boiler.

Can you tell me what the correct pressure should be as it was fitted by the council about 9 years ago without explanation, Although we have topped it up over the years we have never known the correct pressure