IF calculation

I am writing a sheet for my tax return. I am writing the
distance in one cell. In the second I need to deduct 12 KM
for each day, per direction. Third cell needs to return
the value above 100 KM each day. If this sum is lees than
100 KM the formula just has to leave the value "as is".
And is there, by the way, a possibility to include the
value 1,59 per KM < 100 and 0,79 > 100 per KM in the
formula. I can make the first part of the funtion to
return the value > 100 as a result, but how do I get the
formula to leave the figure entered. As I often work from
one day into another one formula needs to be able to only
deduct the 12 KM per direction once a day.

Who can help me on this one?