Quote: "Leave it too late (below 500 feet) in this mk of seat and you gamble somewhat".
The Mk4 seat in the JP/Strikemaster is a zero/90 seat. In other words the seat has been designed to work at ground level at a minimum forward speed of 90 knots. This is in level flight. For descending flight, one tenth of the rate of descent is the minimum ejection altitude. A 2000fpm descent makes the minimum altitude 200'.
The above statement is true of you are descending at 5000fpm.