When you subtract the time between their message and your response so as to avoid the following: seeming desperate, sounding actually upset at them, desperate, glad that someone cared enough to respond to you, desperate, and to avoid sounding desperate.
Text math is done in a number of scenarios such as during arguments with your soon-to-be ex, your parents when you're drunk, or with that co-worker when you aren't sure how exactly they got your number.
I have my response, but I need to wait 7 minutes to send it according to my text math.