I wasn't aware of the online recipe calorie calculators previously, so I'll definitely be using those in the future!
I've just been doing it the 'old school' way. I look at the nutritional information on the ingredients and then calculate how many calories there are then divide by the yield of the recipe.
I have also tried googling "vegan" and "low calorie" versions of certain recipes to help give me a better idea of healthier or low calorie ingredient substitutions, and depending on what I have at home or can/like to eat I then combine the recipes to my liking.
The first batch may be a bit of a 'test round', which may take a bit of tweaking, but altering recipes this way has worked well for me.