Roger,
I would go along the target ppm and work backwards to get the amount of macros that you need.
Targets are:
25 - 35 ppm NO3
10 - 30 ppm K+
1 - 2 ppm PO4
And you are using Ca(NO3)2 to add calcium and the normal macros like KNO3, KH2PO4 and K2SO4.
If we go along Tom's suggesstion of adding 3/4 tsp Ca(NO3)2 and 1 tsp K2SO4 after every WC to your 160g tank, we'll have (with Fertilator):
4.73 ppm NO3
4.74 ppm K
1.01 ppm Ca
Looking at the target values, you are short of
25 minus 4.73 = 20.27 ppm NO3
10 minus 4.74 = 5.26 ppm K+
and 1 ppm PO4
Now you need to divide those figures by 3, because you are going to dose the macros 3 times a week (or daily, then adjust accordingly).
20.27 divide by 3 = 6.8 ppm NO3
5.26 divide by 3 = 1.8 ppm K+
1 divide by 3 = 0.3 ppm PO4
So how many grams or tsp of macros do you need to add to your 160g to get those ppm levels?
Again, with Fertilator, it showed that you need to add
1) 6.8g (about 1.25 tsp) of KNO3 3 times a week
2) BUT with that 6.8g of KNO3, you are also adding 4.34ppm K into your tank. And 3 times a week will add 13 ppm K into your 160g tank. Adding that to the amount that we add after WC, we get a total of nearly 18 ppm K+ in a week.
3) 0.26g of KH2PO4 3 times a week (will also add 0.12 ppm K)
As mentioned by all our Plant Guru, this is no rocket science; but like many others, I would go thru all those calculations just as a note so that I won't stray too much
Anyway, it's great if our guru can confirm my calculations. TIA.