CMS Calculation Dispute - Shared Care Nights & Expenses

[davec2019]

Hi all, I'm hoping for some advice on a CMS dispute. My case was opened on 22 January 2026 and the initial calculation is wrong in several ways.

The issues:

1. Shared care is wrong - CMS have recorded 1 night/week for both children. The correct figure is 156 nights/year following an alternating fortnightly pattern (Week 1: Wed-Thu nights, Week 2: Wed-Sat nights). This places me in the 3/7 deduction bracket. My ex stated 4 nights per fortnight, I confirmed the correct figure of 6 nights per fortnight.
2. My ex's figure doesn't add up - My ex stated 4 nights per fortnight to CMS. However given the established pattern includes Friday and Saturday on the longer week (2 nights she doesn't dispute), the remaining 2 nights in her own figure can only be Wednesday and Thursday. This actually confirms my pattern but understates it by 2 nights - specifically Wednesday and Thursday on the longer week, nights on which I am providing sole childcare whilst she is present at the property but not caring for the children.
3. Expenses not recorded - I pay half the mortgage on the property where the children primarily reside. A CMS advisor (call recorded, 17 Feb 2026) specifically told me to log this as an expense.
4. Disputed nights - The children's mother is disputing the 156 nights. CMS say they need a court order to accept my figure despite me having a comprehensive evidence package with 4 independent data sources (workplace authentication logs, Google Maps Timeline, bank statements and WhatsApp communications) all confirming the 156-night pattern spanning 7 months.
5. Misrepresentation of care - On the Wednesday and Thursday nights on the longer week, my ex is actually present at the jointly owned property but is not providing childcare - she is going out or doing her own activities whilst I am the one responsible for the children. Her nights should not count as childcare nights.

Where I am currently:

• Unable to update shared care online as the case is too new
• Considering a C100 Child Arrangements Order application to force CMS to accept the correct nights
• Concerned the incorrect CMS figure could be used against me in upcoming divorce mediation
• Need to arrange a MIAM before I can proceed with a C100

My questions:

1. Does paying half the mortgage qualify as a legitimate CMS expense variation?
2. Is a C100 the best route given the mother is disputing the nights?
3. How do I prevent the incorrect CMS figure being used against me in mediation?
4. Is it fair that CMS appear to be taking my ex's word without question whilst requiring me to obtain a court order to prove my position? What are my rights here?
5. Can the fact that my ex is present at the property but not providing actual childcare be taken into account?
6. My ex's own figure of 4 nights per fortnight mathematically confirms the pattern I describe - can this be used as evidence against her disputed position?

Any advice greatly appreciated.

[echat11]

Celestine Can you please take a look. Thanks.