Yep as long as it's all 11" and not a mix of 3" and 11" and so forth. You just take the total length of the flex you are using x the watts/foot rating for that flex and you get the total wattage number.

From the Reptile Basics website:

11" flexwatt is 20 watts per foot

4" flexwatt is 8 watts per foot

3" flexwatt is 10 watts per foot (though Rich recommends just calculating as you would for the 4" to give yourself an additional safety margin)

So if you are using say 3 pieces of 11" flewatt - 1 is 1 foot long, the other 2 pieces are say 2 feet long. Your calculations are:

1 + 2 + 2 = 5 total feet of flexwatt x 20 watts per foot = 100 watts total

Here's a great link from the Reptile Basics website - Flexwatt FAQ

http://www.reptilebasics.com/Flexwatt-FAQ-sp-4.html