This workout is what Hunter Allen recommends for testing your FTP. The test is actually supposed to be 20 minutes, but I ride a 10 mile time trial which takes me around 25 minutes. I wish it only took me 20 :-)
2x20 FTP Test
Warm up: 10-15', include 3x30"(30") spin-ups; then 3x2'(1')@ 80-85% FTP
Main Set: 2x20'(2') as 20'@ TT Effort, 2' rest, 20' TT Effort.. NP for the 42' effort is roughly FTP
Cool Down: spin easy for 5-10'