Setting the gap to 0.25mm is a daunting task if you don’t have feeler gauges, so my tip is to use a piece of 3M Blue Tape folded in half as this gives you 0.24mm. Don’t do the final adjust using the Z-axis stop screw but try to get it near, then use the platform levelling screws to make the final tweaks. If you can just slide the tape between the head and platform without it being pinched you have got it right but you will need to do it across the whole table. This assumes you have done all the modifications to the holes and countersinks beforehand, which are critical.