Do you have Thermo DMM at all? If not, its not that easy to "know" where you are in the roast cycle temperature wise. Basically, you need to reduce the heat a little once a vigorous Rolling First Crack (RFC) is under way to reduce the gradient of the temperature rise. Its not a good idea to allow the bean mass to head into a negative gradient at this stage as this seems to suppress the flavour development; almost as though the flavours get set and can not be reinvigorated.
With the aid of a Thermo DMM this is a much easier process to control but not impossible without one. For example, with my Corretto setup, I only have to raise the heatgun about 25mm or so (on average) to achieve an ideal temperature gradient after Second Crack (SC), so perhaps you could borrow someone elses DMM/Probe to help you work out how to adjust your heatgun position/temperature. Good powers of observation and patience are definitely handy attributes to have though mate; the smoother you can make the whole roasting process Im sure will reap rewards in the cup.
Cheers Chris :)