HICBC is much higher when adding medical and dental

Hi there. When completing my self-assessment, the HICBC charge (2 kids full year, £56k) is calculating an amount due of £1743.40. The HMRC on-line estimate is £1319. I thought this difference might be due to the £2139 private medical and dental field but weirdly when I remove that from the self assessment (I know it needs to be there but wait.....) the tax amount due falls to £491.80 Any idea whats going on?

Hi Caseyrybackx,
We would need to access your record to check why the balance calculated is different than expected
Self Assessment: general enquiries
Thank you.