It sounds like what you’re fundamentally after is whittling down the total size of your backlog of tasks, right? It might be best to beemind that directly with a Whittle Down goal where the metric is just [current size of to-do list]. Would you consider doing this via Trello? If so I have some ideas I’ll point you to. Also our HabitRPG (recently renamed Habitica!) integration includes an option for whittling down the size of your To-Do list. If you’re into Habitica’s delightful nerdery then that’s my first recommendation.
But to answer the question more directly, what about two recipes (or zaps, as Zapier calls them): one sends a +1 to Beeminder when you add a task and the other sends a -1 to the same Beeminder goal when you complete a task.
PS: Todoist also has an IFTTT channel and if you want to do other fancy things with IFTTT triggers you might want to check out our IFTTT macros.