A battle on ice

As a DM I enjoy spicy up the environment in unique ways to try and increase the fun and challenge of a battle.  Recently I had the party on a lake of ice.  In order to implement realism, I looked for the various ice related rules in the books.  Some information about the thickness of ice relative to its hitpoints, and adjustments to movement rates could be found, but I had to improvise others.

Ice slows by 50% per page 52 of the DMG II (I think).  This I found in a section about an ice bridge.
I wanted to give each square about 30HP, thus the ice was 10 inches thick.  The ice is immune to cold damage, but vulnerable to fire damage (per the DMG) and thus takes 50% more damage from fire attacks than the die rolls on the fire attacks show. 
The rules that I came up with before the battle were that the ice has DR10/bludgeoning.  Also, if a section of ice takes 30HP it will break; and then each adjacent section (the 4 squares touching it) also take 10HP (which may cause any of them to fail if already damaged).

If you are standing on ice and it breaks, a Reflex DC20 save is required to move to another 5' section of ice, but if your weight causes that section to break then a Reflex DC 30 is required to move to an adjacent section of ice; and if that section breaks a Reflexc DC or 40 is required, and so on.

If you step onto a patch of ice and your weight causes it to break, base reflex DC is 15 (this is different than the ice you are already standing on breaking beneath you).

Each 1HP of ice can hold 50 pounds before breaking.

The temperature was 10 degrees. I had CON checks for cold damage after 10 minutes then every minute thereafter.

If a character fll in the water, I required a Fortitude Save DC5, and the DC goes up by 4 every round. Save for 1HP cold (non-lethal), fail for d4 cold (non-lethal).

After getting out of water characters continue to take 1HP for next 4 rounds unless cure spell applied due to continue cold sinking in.