"The Computation" is a witty and ironic title for this poem. It fits a pattern in which many of Donne's poems are titled, "The [Noun]," like, "The Flea," and "The Anniversary."
A "computation" just means adding up numbers, like an accountant does. And, of course, a "computation" sounds dry and emotional, which is the very opposite of what this melodramatic poem represents. That's what makes the title ironic. The speaker is doing more than just adding up numbers, though he is also doing that. He is lamenting how he has been in limbo since he last saw someone.