I just go by feel. I have tried to learn some systems, but considered it waste of time. I very often use spin on the cueball aswell, so it will change direction completely after 2nd rail, and for those shots you have to use feel.
The best way to practise is to just use the cueball and fire it off on the table for 20 minutes, with no other balls. Use different speed and spin, and see how it reacts.
I have just practised with the cueball lots of times, and it has definitely helped me with kickshots.
Another good advice is to not grab your jumpcue if there is a ball blocking the way. Very often it is much better to get 1,2 or 3 rails to hit, then hide the balls. I use my jumpcue almost only on the simplest jumpshots, where I am almost guaranteed to make the ball and have position. I feel more comfortable to kick back on safeties, instead of jumping.