I think it does matter how your hours are spread I'm afraid. I would never express a p/t employee's holiday entitlement as how many full time days they get off, that would be confusing and difficult to adminster/keep track of for the employee and their manager.
Imagine you work, say, 9-3 5 days a week, but your holiday is recorded in 'full' days. If you wanted to book a day off, you would have to deduct, 2/3 (or whatever fraction) of a day from your holiday entitlement. If you wanted to take a week off, even though you work 5 days a week, you couldn't book 5 days holiday, you'd have to work out how many full days that would be. Holiday for part timers can often be confusing for managers and employees alike and I think this would not help!
Assuming you work regular hours and more or less the same hours everyday, I would always express it in terms of how many days you work. For example if the basic f/t entitlement is 25 days, if you work 9-12 5 days a week, you would get 25 days. This is fair because a Monday off for you would be 3 hours off, while a MOnday off for a f/t person would be 7 hours (or whatever a full day is).
If you work p/t 3 full days a week, you would get 25/5x3 as your holiday entitlement, i.e. 15 days. This is fair because although you get less 'days off' you are getting a full day off each time, and a week off for you would be 3 days off. Thinking of it in terms of 'weeks' off sometimes helps, a week off for someone who works 5 days a week is 5 days off, a week off for someone who works 3 days a week is only 3 days off, even though they may work the same hours, so it's clearer if the holiday entitlement is recorded in these terms.
I agree with Ribena that if your hours are going to be v irregular, it should be expressed in terms of hours off. I think in your letter they probably have expressed it in terms of how many full days it would be, but it is particularly poor drafting that they have not been clear about this.
Once you are clear on how the hours would be spread, ask for your holiday entitlement to be clarified to you in terms of how many days off a year you will get, based on how many days a week you work. If it's irregular, ask for it to be calculated in hours and confirmation of how you should book it.
Imagine your poor manager if he/she tries to keep a record of your holiday if it's 2/3 day for each day off!