I forgot to say: if the amount of money withheld is lower than your average tax rate calculated last year (it probably is) on the value of the RSU, you might want to consider doing a quarterly tax payment. Probably too late now, so you should consider it next time. There's a lot to consider here. For example, if you calculate the penalty and find it is less than you will make in the market as it goes up, you might want to forget about the quarterly payment and keep the money in the market. But like going to Las Vegas and betting, you might lose your money if the market goes down.
