I don't have the figures to hand here but if I'm wrong I'm sure someone will point it out.
Take the total number of eligible voters in Catalonia. Subtract 20% (on the assumption that maybe some of the habitual non-voters would stir themselves for a level-playing-field independence referendum). Your target for a 50% vote in a level playing field referendum is 50% of that figure.
Take the number of actual Yes votes cast last week
Take the 700,000 ballot papers confiscated by the Spanish police while still in their ballot boxes and allocate to Yes the same proportion of these as was cast for Yes in the ballots that were actually counted. Add that to the total number of Yes votes.
Now, consider the difference between the actual turnout (counted ballots plus confiscated ballots) and the notional 80% turnout in a level playing field referendum. Being really, really kind to the No camp, allocate only 10% for Yes. (There is no doubt that a significant number of potential Yes voters were intimidated away from the polling stations last week.) Add that 10% to the total.
At this point you are over 50% for Yes. Whether you're over 55% I don't know, but Madrid is right to keep trying to prevent a fair vote. They'd probably lose.