Is a -900 calorie deficit affective or bad news for safe weight loss (1-2 lbs a week)?

Hi, Sorry I am new to the whole weight loss thing and I'm 26 years old - I think in some ways I'm proud of myself for not feeling the need before but now I am not happy and want to lose at least a stone. I'm not in a hurry. But I can't get my head around calorie deficits etc?

So I weigh 180lbs, I am 5'3 and my job is very light active (think shop keeper without a stool for 8 hours a day) - various apps have said my basic calorie intake should be 1700 to maintain - do you agree?

If I keep an eye on my diet I can safely eat 1200-1300 calories a day without feeling awful. What does this mean? Am I already at a -400 deficit then?

On top of that, three days a week I will walk home (6.5 miles) fairly quickly for my little legs (3 mph?). Websites seem to suggest that with my weight I will burn 90 calories every mile, do you agree? So 540 calories per time? Does that mean on those days I will have a -940 calorie deficit??? I don't get it. So if I have that deficit 3 times a week, that will be -2820 a week + the deficits on the days I don't walk as well?

Is this effective? Can I expect to lose weight steadily doing this for two months? Sorry I don't really understand.