The Magical Number 0.018

to know the diameter of light without getting back to the cone diagram there is a quick formula

Diameter of light = 0.018 × beam angle × the distance

for example if you need the diameter of light for a spotlight of 14° @ 3 meter distance then

Diameter=0.018×14×3=0.756

this formula is correct for beam angle up to 50°

DDR(Diameter to Distance Ratio)

the 0.018 is quite handy but I am using more handy way which I would like to share with you.

I am proposing the  DDR (Diameter to Distance Ratio)

this DDR simple relation between different angle and diameter of light for famous angles