Formula for working out holiday allowance

[Oodhousekeeping]

Dh is in a 'casual' job and accumulates holidays as he goes along.
It's the standard 5.6 weeks if you are F/T. How can I translate that to calculate dh allowance (because dh is crap at remembering to check & I'd like a rough idea!). His shifts vary in length so would have to be in hours.

I'm usually good at these sorts of things but my am preg and full of painkillers

[flowery]

holiday calculator

[Oodhousekeeping]

Fantastic thanks :)