>Is there a specific ratio and method for substituting confectioners sugar
>for regular table sugar in baking or in iced tea?

For information purposes
For beverages as long as you can disperse it properly it will behave
and taste similarly like normal table sugar.
In fact the finer particle size ( as long as its not lumpy before you
use it) will allow it to dissolve faster than coarser sugar for
beverage and other sweetening application.
For baking if you make cakes such as batter types where you blend the
sugar with fat initially, it has inferior performance than the fine
granulated or caster sugar as it does not allow the fat to entrap air
bubbles well during the creaming process.
If that is the case you had to add slightly more baking powder in order
to get the same results ( cake volume)as fine granulated sugar.
Even in sponges, it does not whip as good as the normal sugar due to
the same reason.. In addition it will just dissolve in the aqueous
phase of the eggs instead of helping in air bubble entrapment and
stabilization in the egg foam. .
Regarding ratio, Vox is right the starch filler is only within the 3%
range and not any higher. Therefore for every 100 grams of powdered
sugar you have 97 grams of sucrose and 3 grams of filler not
significant enough to alter ingredient balance in the low ratio cake
recipe; but can be noticeable for high ratio cake formulation..
If you want to use it in equivalent basis in terms of sucrose
concentration better use more powdered sugar than normal fine
granulated sugar.
BTW, For cookies, the product with powdered sugar does not spread very
well and your cookies appear to have restricted product volume/

Finally ,
>I ran out of regular sugar...
>gotta run to the store, but since I have this confectioner's sugar around
>here...
>can it be done?

You have two option, either , to satisfy your curiosity by playing with
it or stretch your legs and run to the nearest store!
Roy