On the 737 700 , fuel from start up until take off is 10kgs/min...hence 200kgs gives you 20 minutes.
If you take off with less than take off fuel (ie you burnt 300kgs due to delay while deicing for example) you ARENT legal to take off.
Yes you may save fuel enroute but it still doesnt make it legal,you may also change your alternate but a proper OFP must be calculated for proper fuel calculation( longest STAR...)
Thats my 2 cents.
