Light requirements are dependant on vessel length
check the box of new lights in the chandlery, they are usually marked for suitable size of boat to comply with International Regs for Prevention of Collision at Sea
reg 22 (c)
Vessels less than 12 metres in length visible range requirement
masthead light 2 miles
sidelight 1 mile sternlight 2 miles
allround lights 2 miles
Luminous range of lights measured in candelas shown in annex I
1 mile range 0.9 candelas
2 miles 4.3 candelas
I have a table that says that a tungsten lamp (as normally fitted in nav lights I beleive) emits 12lumens/Watt. (if a tungsten halogen lampt could be fitted it would emit 18lumens/Watt - less electricity for same performance)
To convert from lumens to Candela you have to look at the characteristics of the reflector because: lumens = candelas x solid angle of emission in steradians.
The solid angle of emission bit is a bit difficult to grasp but is basically a measure of the width/height of a cone of light (easier to grasp if you think of a beam of light coming from a torch). How you work this out for a Nav light I'm not quite sure. Also how you calculate the effect of the lens I don't know.
Probably better, I suppose, to look at the box of a new nav light as previously suggested. I have got a book on lighting if you are really interested though.