As a general rule of thumb, the A320 will enter Alt* when the altitude left to climb/descent equals about 1/3 of the ROC/D. Eg climbing at 1500fpm to FL100 you’ll get Alt* approximately FL95. Another good tip for those new to the aircraft is that if you want to get out of Alt* using the autopilot due to too high a climb/descent rate then you need to use ‘push to level off’. Simply selecting a lower V/S will just see Alt* re-engage after a second or so as the conditions remain the same (high instantaneous V/S and a reducing height to level off altitude). By selecting V/S 0 the aircraft initially ignores the commanded Alt and you can then wind the V/S to a more appropriate rate as the actual V/S reduces. Very useful as a ‘get out of jail’ card to avoid a nuisance TCAS.