Working out my holiday/vacation entitlement I am trying to figure out my holiday (vacation) entitlement.  Can somebody please tell me if I am right?  
I get $27$ days holiday plus the bank holidays (public holidays).  My work year is $1$st of April to 31st of March. Due to Easter moving about that means this year I get $7$ days as bank holidays.  In total this year I get $34$ days holiday entitlement
If I worked full time I would do $7.5$ hours per day and work $5$ days per week $= 37.5$ hours per week.  $34$ holidays days $\times 7.5$ hours a day $= 255$ hours holiday entitlement.  If I was full time every time I take holiday day I would lose $7.5$ hours/$1$ day from my holiday entitlement.    
I don’t work full time.  I do $18.45$ hours per week which is half a week’s work.  I don’t work $2.5$ days though.  I work $3$ days per week and do $6.45$ hours a day.
Based on that information am I correct in saying the following?
This year I get $127.5$ hours holiday entitlement (half of a full time worker). Every time I take a holiday day I take off $6.15$ hours from my holiday entitlement
Or should it simply be: 
I get $34$ days holiday entitlement.  Every time I take a holiday day I just take a day off my holiday entitlement.
 A: I'm fairly certain that the policy on this depends on your employer. You should probably consult your HR department.
That said, let's talk about the math: some of your assumptions appear strange to me. You say that full time is $7.5$ hours per day and $5$ days per week (which would be different from the common standard of $40$ hours per week) but then you also say that your "half-time" appointment is $18.45$ hours per week. $18.45$ hours per week is $49.2\%$ of $37.5$ - do you mean $18.75$, which would be $18$ hours and $45$ minutes? Relatedly, I think your "$6.45$ hours per day" should be "$6$ hours and $15$ minutes per day" (one-third of $18.75$ hours), which is $6.25$ hours per day. Recall that there are only $60$ minutes in an hour. In which case, assuming that your employer measures holiday time hourly, you should have a total of $127.5$ holiday hours from which you should deduct $6.25$ every time you take a day off. That's approximately $20$ days off ($20$ days with about three additional hours left over).
However, I have never had an employer that counted public holidays in terms of hours off - again, you should talk to your employer's HR department or someone else directly familiar with your employer's policies.
