For 600 calories at a 40/40/20 P/F/C split, you want 240 calories from fat (26.6666g), 240 calories from protein (60g) and 120 calories from carbs (30g).
30 grams of carbs is obtained from about 81 grams of Huel, but we’ll round that down to 80g because you’ll likely get some carbs from your chosen fat/protein source.
80g of Huel powder contains the following:
How much protein powder you add will depend on which protein powder you have, but assuming you have a fairly standard whey protein like this one in unflavoured, you’d need to add around 44 grams to get up to 60g of protein. To be more precise, 80g of Huel plus 44g of that unflavoured whey gives the following:
The easiest way to get the fat up is with an oil, as that would be pure fat. So you’d then add about 11g of oil, which would be exactly 11g of fat (99 calories) to get the following:
Assuming 9 calories per gram of fat, 4 calories per gram of carbs and protein, and 2 calories per gram of fibre, the macronutrient ratio of this meal is 37% fat, 40% protein, 21% carb and 2% fibre.
You can play around with the ratios a little if you’d rather get fat closer to 40%, but this is around about the sort of ratio you’re looking for. If using a nut butter instead, this will likely have some calories from protein/carbs that you’ll want to take account of too.