I'm thinking you are a bit close to uke in that replay, i would use around 130-140 just so you have some room to move your arm into the punch. Now, i'm no dafe but i do love making punching replays. The start to that replay looked promising, you are using relax and you have the whole turn thing going on. Where it went a little bit pear shaped is the footing, you didn't get a grip on the ground to put power into your punch. It is possible to do a leap from the ground with a kick, to punch up and decap uke, this could be slightly easier for a beginner. *looks over some old punch decaps* ok now, hitting from below can help decap uke, using the momentum you can just pop that head right off. Ok, i will post some of my punch replays, watch them then have another go. Report back afterwards :]
Another Puch ;o.rpl - That is a good example of a jumping decap, using the kick upwards to put power behind the punch.
Punch Decap.rpl - That is a pretty simple one, not much involved.