I don’t know that it’s too much effort, it’s more that there are decisions to be made. Using the longest word length is just the foolproof method, as it makes sure that all the blanks will definitely have the same width. You can try using a shorter constant width, but the question becomes “what to do with longer words?”
No matter how you fine-tune this parameter though, as you say you won’t have control over the page resolution when your participants take your experiment, so there’s no way to guarantee 100% that no wrap will be introduced.
Jeremy