Using Vin is still a problem for many. I did some calculations and here are the results. The UNO uses the NCP1117ST50T3G regulator to generate 5V from Vin.
Data sheet here.
From the data sheet pages 2 and 8 it appears the maximum power dissipation is about 600mW. The difference between Vin and 5V will be dissipated as heat so dividing the max power by the difference voltage will give the maximum current.
Vin......Difference .......Max current
12V............7V...................85ma
9V.............4V..................150ma
7.5V..........2.5V...............240ma (I use this)
7V.............2V................. 300ma (test voltage page 3 of data sheet)
6.5V.........1.5V................400ma (minimum on page3 of data sheet)
(Anyone know how to prevent deleting extra spaces??)
I use a 7.5V wall wart, which will work even with clones which might have a regulator with a higher dropout voltage.