Work Allocator for employees depending on headcount and shift

I have 78 work items :
1) 28 work items belonging to EST
2) 29 work items belonging to CST
3) 21 work items belonging to PST

Head count varies from 20-26 daily

There are 2 shifts
1) 11 am - 8 pm
2) 1 pm - 10 pm

Using rand and rank function I randomly distributed 78 sites within the categories EST, CST and PST, such that there are no duplicates.

MY PROBLEM:
I'm not able to distribute the random list of 78 work items to employees equally.

The distribution should take place based on shift and working or not.

For example:
If an employee is working 11-8 he should be allotted only EST and CST sites.

If an employee is working 1-10 he should be allotted only CST and PST sites.

If an employee is not working, his work allotment should remain blank.