Y offset not calculated correctly when moving an appointment

[valincius]

Describe the bug
Since the last update changed the heights of the weeks, the calculation for the Y offset is incorrect.

To Reproduce
Steps to reproduce the behavior:

1. Add appointments on a day until the week increases in height
2. Drag one of the appointments to another week
3. You'll see the appointment will not line up with your cursor

Expected behavior
Appointment should stick close to the cursor