Good stuff Mick! ;D
25 Mins. seems a little long. I generally hit first crack at about 8 mins (with 300g beans). 2nd usually follows at 10-12 mins or so. Usually finished well before 15 mins.
First crack starts a little slow, builds momentum and is quite noticeable. The pause between 1st and 2nd is fairly brief but is easy to judge.
1st and 2nd are audibly differrent. 1st is a louder, snapping type of sound. Whereas 2nd is sort of softer but more frequent or as some people say "rolling". Couple of roasts and youll learn to notice.
Its really important to listen though. Theres nearly always the briefest of pauses between 1st and 2nd.
Keep the heatgun on the high setting. Ive learnt that the low one (I use XU1 heatgun) is fairly useless. Just vary the distance you hold the gun. Maybe youre either roasting on a lower setting or holding the gun too far away?
Im on about my 12th batch now and find it fairly simple to pick. Stick with it Mick it only gets easier. :)