How many shares to buy to reach a target average
Short answer: shares needed = current shares × (current average − target) ÷ (target − buy price). The target has to be between the buy price and your current average; otherwise no number of shares gets you there.
Worked example
100 shares at an average of 75.67, current price 68.90, target average 72. Shares needed = 100 × (75.67 − 72) ÷ (72 − 68.90) = 367 ÷ 3.10 ≈ 118.3 shares, about 8,149 in cash.
Why some targets are impossible
Each new share pulls the average toward the buy price but never past it. With a buy price of 68.90, an average of 68.90 or lower can't be reached, and a target above your current average makes no sense for averaging down.
How the cost grows
The closer the target is to the buy price, the faster the required shares grow: halving the distance roughly doubles the shares. Check the cash needed before you commit.
Doing it with AvgNote
In the Avg down tab, enter your holding and the price of your first planned buy, then type a target average. AvgNote shows the shares and cash needed, or tells you the target can't be reached at that price.