Xbox Game Pass Ultimate conversion ratio

I recently wanted to try out the Game Pass Ultimate for Xbox Live. However the wording around the conversion of time remaining is extremely confusing. I have 24 months left of Xbox Live Gold. If I were to use the £1 for 1 month Ultimate deal, would this then reduce my 24 months of gold down to a lower number of months of Ultimate or is it a 1:1 ratio? (I’m a new customer and have never had Ultimate before).

:)