I've always done the 6x5+1 intervals with a 105-108% of FTP target, which is just about the boundary between FTP and VO2max. If I do them right it usually goes something like:
#1: ugh
#2: okay this is not too bad
#3: I am awesome!
#4: uh, how many more?
#5: please kill me
#6: this sucks but this is the last one, c'mon clock, how frickin' long can 5 minutes be?!?

Before I got a PM, I tried to correlate HR but in retrospect I did the first 2 way too hard and the last 2 not hard enough because of the HR lag. You still get good training out of it, but I would pay more attention to RPE than HR if you don't have a PM. Probably the best way to do them without a PM is by speed if you have a nice consistent grade hill.