Coherence length of ruby laser

You have not given enough information to determine the coherence length of the laser. Some ruby lasers have coherence length up to 100 meters. In 1974, 1 meter of coherence length was a big achievement.

You have stated that the Coherence length: $L = dfrac{lambda^2}{Delta lambda}$ .

I will assume that you are using the approximation for the formula:

$$L = frac{2 ln 2}{pi} frac{lambda ^2 }{n Delta lambda}$$

Where $n$ is the refractive index of the medium through which the wave is propagating in and is taken as $1$ for air. For this formula, the value of $Delta lambda$ corresponds to the full width half maximum (FWHM) spectral width of the source.

In optics in general the width of various peaks can be given as FWHM, $dfrac{1}{e}$ or $dfrac{1}{e^2}$ for various different applications. You have not made it clear which is being used for the uncertainty you provided. Despite this, I can say that in optical communication the standard for spectral widths is FWHM.

The literature gives you generally the width and the wavelength of the light emission. If $Delta lambda << lambda$, you can apply the equation $l_c = frac{lambda^2}{Delta lambda}$.

Typically, the width is given in Hz or GHz. You can transform it with the relation $Delta lambda = frac{lambda^2 Delta nu}{c}$.

For your case, Ruby have emission at 694.3 nm and FWHM~330 GHz. You should found something around 1 mm.

NB: A better explanation can be found in the textbook of W.T. Silfvast (ISBN: 9780521833455), where the author compares the coherence length of mercury lamp and helium-neon laser.