There are a number of other simple ways to get a feel for how big a risk this is. You can compute how much your life expectancy is changed by the possibility of getting one of these blood clots, assuming it would kill you immediately with certainty. My back-of-the-envelope calculation results in about 30 minutes if you are in your 20s, but less if you are older.
You can also think of what would happen if you were to give this to the entire adult US population: You'd get 300 people with blood clots, to be compared with about 1,000 people dying every single day. So if using the vaccine can make us get to herd immunity a few hours earlier, it's totally worth it.
The people issuing this recommendation migth think they are acting with an abundance of caution, but the result is going to be hundreds or thousands of additional deaths.