Does FMLA 1,250-hour threshold count “hours worked” or “hours scheduled”?

Q. I realize that to be eligible for FMLA leave, employees must have worked at least 1,250 hours in the past year and have been employed by us for one year. My question is: Can I base eligibility on "hours scheduled" (including hours not worked)? Our Customer Contact Center periodicallly sends employees home early when the call volume is slow. That could mean some employees won't reach 1,250 hours in a year. I don't want to risk a charge that we intentionally sent an individual home so that their hours would fall below 1,250. -- O., NC