A key characteristic of a fartlek session is not paying too much attention to your pace.
Particularly during a base phase, I wouldn't stress paces at all. Run the on-parts by feel, and aim for comfortably hard.
With regards to the specifics you mentioned:
I'll assume we're going by Jack Daniels' definition that tempo = LT = around 60 min race pace. Also, since you're saying that tempo pace is around 30 seconds slower than 5k pace, I'm guessing you're around 17 minute 5k pace.
If that's the case, I think tempo pace is quite reasonable for a 6 x (3 min off / 2 off). But it all comes down to what you're doing the rest of the week.
Regardless, like I said, I would worry less about pace and just run to feel at this stage.