The Chinese Space Station, Tiangong 1, looks very much like a glider i.e. a bit like the US Space Shuttle (solar panels are made of similar materials to Space Shuttle tiles with much less friction), as the German radar images show in the link below, so it most likely wasn't tumbling but yawing from left to right and pitching and rolling just like a glider would, as it comes down to land.
Here's a basic summary of how to project the reentry time of non powered Glider Class reentry vehicles, similar to Tiangong 1, as it might prove useful in the future.
It's probably a little bit easier to look at rough approximates, as I never managed to find out how the US Strategic Command, or anybody else for that matter, calculated their Altitude of Nominal Burst (ANB). The ANB's used below are those provided by Satview.org, so here's the basic methodology.
The basic rule of thumb is that when the natural descent becomes a Pythagorean right triangle with a ratio of 3:4:5 over 8 days. i.e. it drops 6km over 8 days, the time to atmospheric reentry is approximately 1 kilometre per day until the average ANB (over those 8 days) is reached plus an adjustment factor.
I made rough plots and a projection on the link below on March 1, and wasn't sure how recent the data was as the Satview.org UTC timers (and the ANB) were all over the place for most of the reentry, so the end figure I used was 252.5 - 220.25 = 32.25 days - an adjustment from my last plot (i.e. 09:00 UTC March 1), because I didn't know how accurate the timing/altitude was.
Note that my figures don't actually reflect the eventual altitude of atmospheric reentry (ANB), they just indicate the time of atmospheric reentry based on the last data point, that satisfies the criteria in bold, and the average ANB projections over the previous 8 days. Even without the timing adjustment this projection was much more accurate than the forecasts by the US Strategic Command and the European Space Agency (ESA) for most of March.
The eventual atmospheric reentry time given by the US Strategic Command via Satview.org was April 2, 2018 at 00:16 +/- 1 minute UTC (below) and China Manned Space give the reentry altitude as approximately 132.75 km (below).