[dean]

If I add calcium to the mash and sparge to raise the RA to a level high enough for a very dark beer having a pH of 5.4, but it puts the calcium outside the window of 50 to 150 ppm (like 300ppm?)   What effect does it have on the finished beer?   

[ndcube]

I assume your adding it as chalk.  I would just add it to the mash.  In my experience sparging with water that contains bicarbonates raises the pH to high.

[dean]

Yeh, it would be chalk and when I add to the sparge I actually add it to the mash again just before I add the sparge water, stir and let it sit several minutes.

I'm just wondering how having 300 ppm chalk would effect the taste etc. of the beer?  If you look at Dublin's water its within range on chalk being only 118 ppm yet its quite low on other minerals and has a high RA.  

[Thirsty_Monk]

What effect does it have on the finished beer?  

It might taste too chalky.
You could also use Baking Soda to increase your Mash pH. (It will also increase your water hardness)

[Beertracker]

If you're using chalk, you'll definitely need to add it directly to the mash because of the solubility factor, but avoid over alkaline water because it can lead to undesirable phenolics depending upon conditions.  +1 on combining it with baking soda to reach your target pH, as at least you'll be adding some sodium to help accenuate the maltiness. I guess it really depends on what kind of "dark beer" you wanting to make?