The Dashboard will report on all 100 contacts initially. T̶h̶e̶ ̶d̶e̶l̶e̶t̶i̶o̶n̶ ̶i̶s̶ ̶a̶ ̶s̶e̶p̶a̶r̶a̶t̶e̶ ̶a̶c̶t̶i̶o̶n̶.̶
Edited: The Dashboard will report all the contacts initially, but will adjust for the deletions when that statistic is forced to do a complete recalculation. In practice this means that the contacts who have opted in and that you have deleted TODAY will still be in the count for the "Today" time frame most of the time. But when we recalculate the metrics for week to date, last 7 days, last week, etc. you will see the net number of additions minus the deletions. We can't recalculate after every contact deletion because of the massive database activity that would cause, but we typically will recalculate based on other "triggers" in the system such as the calendar day ending, a database update, etc.
In your case, can you design the metric to count only valid contacts? For instance, if you are deleting contacts who do not have a "@" in their email address, you could put a condition in the metric to only count those contacts WITH the "@"