Ask what type of dyno he is using. Ask what type of correction factor he is using. There are two: SAE and STD (sometimes called STP). STD shows higher results than SAE. Mustang Dynos use only SAE. Either ask what the raw numbers are or else find out what the correction factor is. The correction factor is used to standardize numbers from different locations. It is a multiplier. A correction factor greater than 1 (such as 1.02) means your numbers were corrected upward 2%. A correction factor less than 1 ( such as .98) means your numbers were corrected downward by 2%. He might check your air-fuel ratio. If he does find out what it is.
Since you are baselining your car you need to know how the horsepower is calculated. If he uses STD correction for your baseline, you then mod the car and get a second dyno with SAE correction you will be disappointed. If the reverse happens you will be reporting on the forum some impossible but awesome horsepower gain you made. You will then be disappointed when everyone tells you it wasn't possible. If you baseline in STD then you need to check further improvements with STD. If you baseline in SAE the same is true.