Calculating ICM...Newbie Question

I have been observing dethgrind's post for calculating ICM in the FAQ and it seems like a pretty long problem to calculate out when your in the middle of a tournament with a time limit to act. I think I wiould have a hard time inputing these numbers and doing all these calculations when playing in one tournament much less 4 tabling like I usually do. Is there something I missing here or does it just take practice to be able to do these type of calculations on the go...i would think it would be impossible to 8 table while doing this...thanks in advance...