Dear Wedgewood,
The general guideline is to start from a single digit bodyfat then bulk to the point it becomes 15%. Once you hit 15% bodyfat, it would be time to diet down to a single digit or maintain it at 15%, depending on what the circumstances are.
When going into caloric excess, nutrients are partitioned in favour towards lean mass provided you are lean. If one were fat, nutrients would be partitioned in favour of fat than lean mass. This means that someone with 9% bodyfat will gain more muscle than fat compared to someone with 19% bodyfat who will gain more fat than muscle, for the same calories.
Also, unless one were on steroids, natural weight-gain occurs at a rate of around 2lbs per week. Anything greater than that and you know the fat:lean mass ratio is getting bigger, which is no good.
Keeping this in mind, we assume you are of single digit bodyfat. Find out your maintenance calories, then add 200-500 calories per day. Weigh yourself at the end of that weak to check weight gain. View yourself in the mirror too, to see if any visible gains in fat is apparent. If no weight is gained, then add another 200-500 calories for the next week and check again. Keep doing this until you reach the goal of gaining 2lbs per week. At the same time, track your bodyfat to make sure you do not exceed the 15% ceiling.
Godspeed, and happy HSTing
