I lost 3st.
Work your calories out over the week, so that you get a reasonable calorie deficit, approx 500 per day is about a lb a week.
1200 on other days is too low, unless you are very small, as averaged that's less than a 1000 cals a day & that can lead to metabolism issues.
I normally only comment on my own thread but I was a member of the Fast Day forum & we had so many members who basically starved. You lose muscle in preference to fat on calories too low, end up lighter but with worse body composition & it's not sustainable. Count cals on fast days, eat reasonably on the other days.
Done properly, it's an excellent way to lose fat & has a lot of other benefits.