P51 When calculating the required amount of off-the-job training, the apprentice’s statutory leave entitlement should be deducted. Employees who work a 5-day week receive at least 28 days paid annual holiday (this is the statutory leave entitlement referred to in this paragraph and is the equivalent of 5.6 weeks of holiday). You should pro-rata this for part time workers. It should be clear to all parties how you have worked out the amount of off-the-job training required to comply with paragraph P50 and you must record this in the evidence pack.
(my emphasis) So you just pro-rata 28 regardless of actual where you have part timers. The key thing is to make sure it’s the same for all of the learners in this position.