According to the instructions "6g of the powder increases alkalinity of 100L by 2.5dKH", but you might want to compare with the instructions on your box to ensure we are talking about the same product.
40gal ≈ 151L (when accounting for substrate and rocks it will be less, maybe 120L or so).
So
6g will probably be more like
2.1dKH in your tank.
If you find out what your daily alkalinity consumption is, you can simply use the following formula to find out how much powder you need to add daily (you should still verify with tests regularly):
[required powder]g = 6g ÷ 2.1dKH ⋅ [daily alkalinity demand]dKH
[daily alkalinity demand]dKH = ([measurement 1]dKH - [measurement 2]dKH) ⋅ (24h ÷ [time between measurements]h)
But I can help you with the calculations once you have taken the two measurements :)
I'm not really a fan of adding undissolved powder to a tank, especially if I don't know what this powder is, so I suggest you take a look at products that are actually designed to supply a reef tank with
all it needs (calcium, magnesium, alkalinity, trace elements). "All for Reef" is only one of many possibilities:
- Original Balling (3 solutions, Calcium, Alkalinity, Magnesium+Trace)
- DIY Recipe if you don't want to use Tropic Marin or other brands
- ATI Essentials
- BRS 2-Part
- Aquaforest 3-Part
- Reef Zlements
- RedSea (Foundation, 4-part, or 7-part)
- etc.