It's been a long time since using a slotted disc and remember having similar problems and think i just increased the diameter of the disc and also made the slot wider (may be double the calculated ) as the calculated value was too slim. You can make the slot quite wide, so use the calculated as a quide to start with. When fooling around, I used disc's made from a CD which was painted black and just widened the slot until it would read 3450. You can also cut numerous slots and see the affect just by putting black tape over the slots not being used. The above is easy and cheap to do. Sorry I can't be more definitive.
Good point on the response time Hood. Before using the Halls sensor i used a read switch and after reading up on that switch found that is would not be fast enough.
RICH