Plunging fire

[piersyf]

Doubling the deck value implies a strike angle of 60 degrees from the vertical, only just what might be called plunging fire (actually approaching ricochet limits depending on shell nose shape). A multiple of 1.5 is probably better (closer to 45 degrees, near maximum range for the gun).
Rounds do lose velocity over distance, but do not ever get to 'free fall'. I don't think any significant large naval round would ever fall below the speed of sound, so if you consider a 200kg shot moving at around 1500km/h as 'free fall' you might get some wrong ideas on the effectiveness of armour.
BTW, the people who design direct fire guns try REALLY HARD to ensure the rounds stay above the speed of sound throughout their normal flight because rounds lose stability when moving through the trans-sonic region, and hence lose accuracy. May not be such an issue for rounds above say 12".
Don't reduce the value of the belt. It is being struck at an angle as well.
My 2 cents for a REALLY simplistic approach? At normal ranges, use the belt values as they are. Deck hits would be at such a high impact angle you may as well ignore them (a hit is a hit against the side).
At mid range (angle of impact 30 degrees from horizontal) belt armour MIGHT increase by about 1.15 (depending on whether the ship was designed with sloping belt armour... some were) and the deck value doubled. At maximum range, strike angle near 45 degrees, multiply by both by 1.5.

[piersyf]

As a simple compromise it could be a lot worse... if it works for you, try it and see. Given how low ships sit in the water when they're armoured (beam width improves flotation and stability when packing the iron) you could consider only 1 in 6 hitting the belt at long range and 456 to hit the deck... or maybe just at extreme range (and a 1.5 multiplier for the deck)... it won't really impact the game that much as the chance of hitting at those ranges is pretty low.
This might be worth a read for you:
http://www.navweaps.com/index_tech/tech-069.htm