Average-down formula
If you hold q units at average price p and buy q′ more at price p′, the new average is:
avg′ = (q · p + q′ · p′) / (q + q′)
When you enter a dollar amount instead of a quantity, q′ = spend / p′. A lower buy price pulls the average down; how much depends on size. A $500 add into a $50,000 position barely registers. Matching your existing cost basis in dollars at a 40% discount moves it a lot.
Break-even after the buy is simply the new average. If the asset is still below that line, the percentage bounce required is (avg′ − mark) / mark. That number is often larger than people expect, which is the point of watching it live.
Averaging down is not automatically smart. It concentrates risk in a name that already hurt you. The calculator does not know whether the thesis is intact — only what the new entry would be if you add size.