Hi Wavey,
The recommended ratio of huel to water is 1:5, e.g. 100g huel on 500g water. 100g of huel have 410 kcals. The measuring spoon you get with your first order contains about 38g. (Weighing is more exact, as the volume of a powder can vary.)
So if you calculate your daily demand from your weight, height, age and activity, you can calculate, how much huel you need per day to lose weight and divide that through the number of meals you want to take. Then that is your meal plan.
There is also an excel-sheet, which can do all this work for you at The Mathematical Diet (HuelForecast-Generic.xlsx). I personally use it.
Yes, the huel powder is a total meal replacement, but you may want to ease into it by replacing 1 meal for some days or a week at the beginning, then 2 and then go up to 3 or 4. Makes it easier for the intestinal bacteria to adapt to the new type of food - especially if you are used to a diet low in fibre now.