So at a CYA of 40, your minimum acceptable FC level was 3ppm.
You added 4.2ppm, so your CYA is now ~44.2. Your minimum acceptable FC level is now 3.315ppm.
So let's say your daily FC loss is 4ppm in the Summer, which is not uncommon. You'd have to add the 1.5lbs basically every day. But, uh oh. If you add 1.5lbs every day for 5 days, you just added another 21 CYA. So now your CYA level is 65.2 and your minimum acceptable FC is 4.89.
So you follow through with your original plan, use Liquid Chlorine on the weekend to "shock" and get up to your target. Next week, back to dichlor.
At the end of Week 2, now, you're at 86.2 CYA, and your minimum acceptable FC is 6.465.
I hope you can see where this is going. Four more weeks go by, your CYA is now 170. Your minimum acceptable FC level is now 12.765ppm.
Keep in mind these are minimums. Your actual target levels should be 4+ higher than these because you need to make sure you
end the day above the minimum.
In the end, you end up either:
- With algae, because you didn't keep up on the increasing minimum FC levels.
- Using increasingly ridiculous amounts of supplemental liquid chlorine anyway just to keep bumping up with the increasing minimums, in addition to the dichlor you're also using.
- Having to drain and refill 50%+ of your pool water.
- (And... given that you are concerned about your TDS levels [for whatever personal reasons] -- the ever-increasing minimums and gallons upon gallons of liquid chlorine you end up using to keep up with those minimums, mean your TDS actually probably rises even more than it would have if you had just stuck with the appropriate amount of liquid chlorine and avoided dichlor/trichlor products.)
I'm used to doing more work than this. 
