The crossover point is the moment your ISA pot matches what you still owe on your mortgage. Once you reach it, you could ā if you chose to ā cash in the ISA and clear the debt entirely. Whether that's the right move depends on your circumstances, but knowing when you get there is useful.
One thing surprises most people: how much you put in each month matters far more than the growth rate you achieve. A higher return helps, but you can't control the market. Increasing your monthly contribution by even Ā£50 typically moves the crossover point more than pushing the assumed growth rate from 5% to 8%. The contribution is money you're actually committing ā the rate is an assumption.