Tulip (Darwin hybrid, triumph) is planted 6–8 in deep and 4–6 in apart. Closer uses the low end of that spacing range and Wider the high end.
Show the math
Bulbs per sq ft = 144 ÷ (spacing in inches)²
Bulbs to buy = area × bulbs per sq ft × (1 + spare %), rounded up
Depth rule of thumb = 2 to 3 × bulb height