Don't forget that you type of ice, and how you have your towers set will affect not only the amount of the ice needed, but the taste of the product.
Since (most) towers have a limited about of chilled product to dispense, your soda, will melt the ice as it hits the cups. You might want to have your towers set a "bit" syrupy and then when the product hits the cup, it is a perfect tasting drink.
The ice you use will also have an effect. Cubes don't melt as fast, flakes, (which IMHO are the best) have a larger area exposed to the drink, and will melt faster.
Why do I like flaked ice?
Reason One: When the drink is gone, its easier to munch on flakes, and eat them faster. Once the glass is empty, they may be more inclined to go back and get another
Reason Two: Instand Sno-Cones. Just purchase a few flavors of syurp (I like Ricos) and bamb! Sno-cones, no effort! You can also use the sno-cone flavorings to upsell your coke sales. (Don't carry Cherry coke on your tower) Market this as your "Olde Fashoned Sodas" and charge 25 cents extra for an oldfashioned. Cheery coke, Vanilla coke (good now that Coke is selling it) Cinniamon coke sells well in the winter and what my employees have lovingly called a "Tidy Bowl" Mello Yello and Blue Rasberry.
If you can upsell just a few of your customers on the "Olde Fashioned" your per head will crawl upwards!
Sorry I got off the subject, but its good information to share...
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