sql function to round to specified increment

here is some sample data from a table I have named RoundInfo (ms sql server 2008)

Floor         Ceiling           percent    DollarIncrement    
100                 200                 5                 1
201                 4001               7                 5

I am trying to write a sql function that will take in a parameter that will be
an integer amount


The function will do the following:

1. determine the range the parameter value falls into  (the ranges are set up so the input will definitely fall into one of them)

2  for the range determined above, add the floor and ceiling together and divide that sum by 2

3. multiply the above result by the percent

4. take the result from the result above and round based on the increment


For example,

if the parameter value passed in was 120


since 120 falls in the first range,  the function would do  (100 + 200)/2 = 150
then 150*.05 = 147.07


then since the round is set to nearest 1  increment  the  function should return a value of 8
(if the value were 7.49 the value returned should be 7)

Likewise,

if the parameter value passed in was 400

since 400 falls in the second range,  the function would do  (201 + 4001)/2 = 2101

then 2101*.07 = 147.07
then since the round is set to nearest 5 increment function should return a value of 145 (since 147.07 is closer to 145 than 150)

(if the value were 158 then the value returned should be 160)

Hopefully this makes sense