If you are using the box store kits, get another one...unless you are 5 minutes away from them. Although they say 400 or 500 sq feet (depending on whose you choose) it is less than you will need to do the job correctly.
If you are using the suppliers here, they do a MUCH better job of estimating (Knowing) how far their product spreads and what you, the user, will run up against in doing the job correctly.
Prep is the Key....don't even think about scrimping on this. Prep includes temp, humidity, condition of the floor, taping/masking and the like. Not just doing the floor.