If you’re doing IF for weight loss then the numbers you need are your target weight, current weight, maintenance calories for zero weight loss (Basal Metabolic Rate BMR) and rate of loss (how long you want to take). I usually use this to figure it out but but Omni is good too.
To figure the calories in a meal there a variety of apps that calculate an estimate if there are no nutritional facts included. You would want to do this for each meal if you’re counting. I stopped counting because for me it led to more issues and I focus on eating healthy meals rather than micromanaging calories since I have a short window.
1 pound is roughly 3500 calories. For example if you want to lose 1 pound in a week you could decrease your BMR by 500 (3500/7 days). If you want to lose 1 pound in 2 weeks then you could decrease daily calories by 250 (3500/14 days) etc. Take your current weight and subtract the goal weight. Multiply this number by 3500. You can then divide this by the amount of days (rate of loss) you want to do it by: Current weight-Target weight=Goalx3500=Total calorie deficit needed for target weight/duration=suggested daily calorie deficit ie 150 current weight-130 target weight = 20 pounds. 20x3500=70000 calories. 70000 calories/120 days=-584 calories daily deficit.