not sure how it devalues your ex's 40% You give your wife half of the property as a tenant in common. This means that she owns 50% of the house. you own the remaining 50%. When the house is sold, your wife's share is 50% of the sale price. Your ex wife's share is 40% of the profit on the sale which is always going to be less than your share of 50% of the sale price. My understanding is that the caution is, as I said, a red herring. The agreement that you have with your ex is IF AND WHEN the house is sold.
But, as i said, you are best getting legal advice about whether your wife could force a sale.