At present, there's no way to edit the calories burned; however, you can tweak the activity to more accurately reflect what your HRM says. I would keep the time the same and tweak the speed for a relative number of minutes each
For example, when I used to play tennis, my HRM said I burned less than FitDay calculated, so I would enter, for an hour, 40 minutes singles tennis and 20 minutes doubles tennis. So possibly you could do 4.0 mph for X number of minutes and 4.5 for the remainder of the half hour, tweaking it till it balances out.
But I agree, it would be wonderful to customize an activity. That was in discussion for a while. Maybe someone will let us know if it's still an option on the table.
And now that you don't have to be perfect, you can be good.