What is the percent difference of body fat percent calculation before working out vs. after working out?That depends on the intensity of your workout, specifically how many calories you burned, as well as how long after your workout you are measuring. Your body will still be burning calories for a while after a very intense cardio or resistance training session (up to several hours) before it returns to normal metabolic rates.
Cardio tends to burn more calories during the exercise (due to aerobic cellular aspiration), while resistance training burns more in the post (as well as raising base metabolic rate).
At most you're talking a fraction of a percentage point (e.g. you were 23% after, you may have been 23.5% before) assuming your body only burns fat during exercise (protip: it doesn't) in a very intense hour long cardio program. If you are weight training to build muscle, the difference is going to be negligible immediately after, but comparable 4 - 5 hours after your workout (and considerably higher in the days and weeks ahead as your body adds muscle).
I like to take measurements before working out, personally, but in most cases it shouldn't matter so long as you stick to a fairly regimented exercise and diet routine and measure yourself at the same time every day. Since our circadian rhythms regulate things like water retention and base metabolism, it is also important to try and keep your workouts and measurements to the same time slot every day to improve the accuracy of your measurements.