Slit separation, width, wavelength and screen distance set the fringes. Compare the formula with a Huygens simulation.
The science
Two-slit fringes. Waves from slits separated by d reach a point at angle theta with path difference d sin(theta). Bright fringes occur where d sin(theta) = m lambda, spaced by lambda L / d on a screen at distance L (small angles).
Diffraction envelope. Each slit has finite width a, so the fringes sit under the single-slit pattern sinc^2(alpha), alpha = pi a sin(theta) / lambda. The full intensity is I = I0 cos^2(beta) sinc^2(alpha) with beta = pi d sin(theta) / lambda; the first envelope zero is at lambda L / a. For N slits the cos^2 term becomes (sin N beta / (N sin beta))^2, giving sharper principal maxima.
Numerical check. The panel also sums 48 Huygens secondary sources per slit with exact path lengths and 1/sqrt(r) cylindrical spreading; its intensity matches the analytic curve and conserves energy (two slits pass twice the power of one).
Simplifications. Scalar waves, narrow slits in an ideal opaque screen, monochromatic light, Fraunhofer limit (L much larger than d^2/lambda). The animated field at left uses a reduced d/lambda ratio purely for visibility; the plotted fringe screen and charts use your real numbers.