Sources say to prune it right after the flowers have finished and no later.

I used to prune the deutzia I had after flowering too. sadly mine died of age and honey fungus :o(

Agree - Deutzia shouldn't be pruned early in the year, but as soon as the flowers are over - pruning it back now means you won't get any flowers this year.

Thanks everyone. I'll leave it as it is (it's only 2 feet high anyway), and know better next year.