Not a no-no at all as far as I know. It was explicitly recommended in The Mind Illuminated. It just needs to be an alarm that you'll hear when it's done so you

Not a no-no at all as far as I know. It was explicitly recommended in The Mind Illuminated. It just needs to be an alarm that you'll hear when it's done so you don't have to devote any attention to it, you just focus on meditating until it goes off.