Hi, welcome to the forum.
Firstly, going from 0 to 100% Huel is not advisable. Many peoples bodies including my own had to adjust to Huel. Legend has it that the better the diet before taking Huel the less of an adjustment there is. My own symptoms were mainly gas and heartburn but they got better over time and I also reduced the amount of Huel per portion which helped too. Now I don’t get any symptoms. I would advise replacing lunch for 5 days and then see how that works out.
The rule of thumb for weight loss is to have a net deficit of 500 calories per day to lose 1lb in body fat per week. Net means after exercise. The average target for daily calories is 2000 and I can assure you that only eating 1500 is tough but doable but I wouldn’t want to do less. Eating 2000 and doing 500 calories of exercise is easier. The actual figure for you maybe different and there are some calculators online which may give you something to aim at but working it out over a number of weeks balancing hunger and weight-loss will ultimately be your best option.
There is also something called MyFitnessPal which has a website and an app. This allows you to enter your personal information and keep a record of what you’re eating, how many calories you’ve consumed, track your weight loss etc. The app will let you enter what you’ve just eaten into that day and help you keep track. I’d recommend setting it up using the website not the app.