Nomograph understanding help

I have analysis of my water:
Ca: 53 ppm
Mg: 25 ppm
HCO3: 260 ppm
Alkalinity as CaCO3: 213 ppm
pH: 8,5

And I did used nomograph from John Palmer to determine what beer best suite to water I have. And I get Residual Alkalinity as CaCO3 about 165 ppm and Mash pH about 5,95.
And I don't understand the mash pH meaning. I am confused because pH from Analysis is 8,5, in nomograph I get pH 5,95 and as I know I should end up at the end at 5,2 to 5,6.
So I would kindly ask someone to explain me the meaning of mash pH I get from nomograph.
Is this pH I get when I put barley malt to the water?
Is this pH something I should have before mashing - should I lower the pH prior mashing?

Thanks in advance.

With apologies to Martin and AJ for probably butchering the science a bit...

The pH of your water is fairly meaningless - what matters is how well it is buffered, i.e. how much it resists pH change when you mix the water with the grains. Your water has a lot of carbonates in it, hence the high projected pH by the nomograph (which I am not sure how accurate that is). If you want to use that water, you might consider diluting it with RO water or using phosphoric acid to break through the buffer and drop the mash and sparge pH down.

Personally, I would use RO water. It's a great blank slate and easier to deal with.

I understand that and that is why I would brew dark beers like stout and porter. And what I don't understand is what does it mean pH I get from nomograph.
as I know I should get ph 5 - 5.5 when I end mashing or when I add yeast - right?
And I have water with pH 8.5 and I get pH 5.95 from nomograph. So does it mean I should lower pH from 8.5 to 5.95?