Average measure for first n values

[#1]

I am currently monitoring average closing time for tickets month wise. However, I need average closing time for first 100/first quartile of tickets month wise.

For example:
ID     Open Time   Close Time    Month

  1        10                 20                Jan

  2        20                 25                Jan

  3        20                 30                Jan

The average closing time (Close time-Open Time) for first 2 tickets for Jan would be 7.5

I tried this using set expression but that didn't work.

Thanks