Hi Stan!

Stop adding baking soda. It's not doing you any good and it's raised your Total Alkalinity into a very,very high level.
I don't know how 6 mg/l translates to ppm though this converter I found seems to indicate they are almost the same thing. So your FC measure tops at 6...and you need to get it 25!
Try diluting 3 parts of steam distilled water to 1 part of pool water and see where the meter reads it--then quadruple the reading. If it doesn't hit the "HIGH" mark you need more chlorine.

Here's what I think of your pool guy's advice: Most of it's not going to do any good.

CYA of 100ppm means you need to MAINTAIN a free chlorine level of between 8 and 15ppm, and you need to shock at 25ppm. Lowering your FC before your algae is dead is not going to work.

Pool guy's advice:
While sunlight will break down chlorine, when CYA is over 100ppm, it won't do much, especially since you are at such a high latitude that the sun is much weaker. CYA is like sunscreen for chlorine, and you've got an excess of "sunscreen". It's also NOT what you want to do re: algae.
Brush the walls and floor of you pool to knock the algae loose. Yeah, that's good advice. Do it every day, but if you aren't going to raise your chlorine it won't help.
Fresh water won't hurt, but the only things YOU should want to dilute are you high CYA and TA levels. To cut your CYA in half you need to replace half your water.

The first won't help (probabaly won't hurt), the 2nd is good, and the last two are like trying to dig a coal mine with a teaspoon. Gonna need a LOT bigger shovel!

http://www.unitconversion.org/concen...onversion.html