If you want to get an idea of how visible your laser will be at a given distance, you can do this easily. I'm no mathematician so forgive me if any calculation is flawed.

You can use this equation d= (b + Dm)/1000

You need to know:

Initial beam diameter in meters(b)

Divergence of laser beam (D)

And the distance you are measuring in meters (m)

For example, my PGL has an initial beam diameter of about 2mm, approximate divergence of 0.84mRad and I want to measure at 9.4 miles (15208 meters) (distance from my house to nearest mountain).

d=0.002+0.852(15208) = 12957.216/1000 = 12.957216m (42.5ft)

So at a distance of 9.4 miles the beam diameter of my PGL will be about 13meters across.

If you want to go a bit further, you can check this table for the Luminous efficacy at whatever wavelength laser you are using:

Luminous Efficacy
Both photopic (light adapted eyes) and scotopic (dark adapted eyes) conversions are listed.

To calculate approximately how many lumens your laser is outputting, take the lm/W value for your specific wavelength and multiply it by the power of your laser in Watts.

Again, for my PGL:

Photopic: 588.746 * 0.65W = 382.6849 lm

Scotopic: 1378.7 * 0.65W= 896.155 lm

I'm not sure how accurate this Watt to Lumen vs. Wavelength conversion is but it seems about right.

I hope this is useful info for somebody.