Break-even units are fixed costs divided by the margin on each unit. The margin is the price minus the variable cost. Break-even sales dollars are the break-even units times the price. If the margin is zero or negative, there is no break-even, because each sale loses money or makes none.
Fixed costs of $12,000, a $40 price, and a $25 variable cost leave $15 of margin. Break-even is 800 units, or $32,000 of sales. The 801st unit is the first one that contributes only to profit.
A changing price, a step-up in rent at some volume, and taxes on profit are not included. The fixed cost is the single number you enter.