Need help with holiday pay (England)

I have a claim with ACAS which is currently in early conciliation. My employer was wrongly paying me on a self-employed basis, so I am claiming for all of the backdated employment entitlements.
For the holiday entitlement, I was asked to provide a monetary amount and calculations. I used the gov.uk holiday entitlement calculator which I felt was the most fair and reliable source as I wanted to avoid comparing manual calculations with the other party and arguing over discrepancies, but alas, they are now arguing with me over discrepancies.

I started on 20th March 2023 and my employer's holiday year began on 1st April 2022. For the period 20th March to 31st March, the gov calculator gives me an entitlement of 12.7 hours (based on working 25.25 hours per week, 5 days a week).
The employer is disputing the figure with a much lower entitlement for that period, which their employment law advisor has manually calculated. There's £100 difference between my figure and their figure. They also have a second discrepancy for the 2024 holiday year, which is again less than what the gov calculator says by literally £3 (yes they are complaining over £3). Bottom line is I have no idea why their manual calculations aren't matching the gov.uk calculator, especially for the period of 20th March to 31st March. They are accusing me of entering the wrong information on the gov calculator, but I have sent them all the screenshots and told them to enter the same information and they will get the exact same results it gave me.

My employer is now pressuring me to agree to their lower calculations and I don't know what I should do. It's disheartening that they've purposefully been breaching employment law to avoid paying employment rights and now they have the nerve to argue with me over a £100 difference. Extra disheartening too because I've not asked them for any extra compensation for the added stress and many hours spent of my personal time sorting this all out, when this was their legal responsibility in the first place. Typical really - no accountability and apologies given, yet continuing to have no shame even during my claim.