Shovel has the right answer, but you sound kinda-sorta close. I did 3:43 Saturday and ended up with a 274 TSS, but that was at an average of low tempo level for me. Still, testing will always yield better results than a SWAG.

A relatively quick way to test is the way Carmichael Training Systems does it. Pick about a three mile course you can ride without disruption. Do your favorite thorough warmup, and then go as hard as you can for eight minutes and record your power. Ride back to the start point at a recovery pace and then do it again. Record the power for the second interval. Take the higher of the two and multiply that by 0.9, and that's your FTP.