What is a simple rule of thumb for calories per kilogram of body weight?

For dogs roughly in the 15 to 30kg range, dividing the calculated daily calorie target by body weight in kilograms tends to land somewhere around 55 to 65 kcal per kilogram for an average neutered adult at normal activity.

This figure is simply the resting energy formula's output already divided through by weight, which happens to land in a fairly narrow band for mid sized dogs specifically.

Because true energy need scales with weight to the power of 0.75 rather than in a straight line, a small dog under 5kg needs considerably more than 65 kcal per kilogram, often 90 kcal per kilogram or more, while a giant breed over 50kg needs considerably less, often closer to 40 to 45 kcal per kilogram.

Using the flat 55 to 65 figure for a very small or very large dog will give a noticeably wrong answer, running too low for small dogs and too high for giant ones.

For any dog under about 10kg or over about 40kg, use the full resting energy formula rather than this flat rule of thumb, since accuracy matters more at the extremes where the flat figure diverges most.

For a quick sanity check on a mid sized dog's calculated amount, this rule of thumb is a reasonable way to confirm a number is in the right ballpark before finalizing it.