Apportioning household expenditure for gifts out of income

Hi all,
I have a married couple, who are both retired and receiving State Pension & DB pensions/annuity. Income split is broadly £35,700 for the wife and £16,700 for the husband.

They are very thrifty and spend much less than what comes in. The clients have completed a detailed expenditure breakdown which suggests surplus income of £14k p.a. and they have evidenced that their cash
accounts have grown by £1,000 per month historically for multiple years.

They are already making gifts out of capital and there is little likelihood of anything changing so we have discussed setting up gifts out of regular income to be made to their daughter of £1,000 per month.

Ideally, these gifts would be from the wife as she has significantly more income and is also several years younger than the husband. However, I want to help them document this clearly and correctly and am struggling to find a clear answer to how the IHT403 form could be completed on an individual basis when one of them dies. The issue is that all of their expenditure is paid from one joint account, and they've been unable to break any of their lines of expenditure down into individual costs.

I can't see any clear methodology from HMRC explaining how to break down pooled expenditure, and am unclear whether joint expenditure should be split 50/50 when documenting , or whether they could reasonably be split in proportion with the income each generates i.e. 68:32?

Thanks for any input.