Its unclear whether you mean in the ceiling or under the units. If underneath, then 60-80 cm apart seems to me to look good. You won't need any more than 20W under here, both in terms of the brightness it creates and the issue about dissipating the heat from the bulb. If you mean in the ceiling than 80-100cm seems OK to me but it depends on the size of your kitchen. Try holding a couple of units up there to judge the effect. 50W bulbs on a dimmer switch works quite well.
hate to be a spoil sport, goody two shoes here, but it is now illegal to do any electric work in kitchens and bathrooms unles you are are a part p registered elctrician. also all works has to have certificate of compliance.