My approach would be to generate the "points" you need for the line equation, given your slope and time parameters, in Excel. Then print a CSV file of the results, then copy the CSV file into your program as a table. It'll take some minor editing to generate your table but no big deal, and get the values from the table with Lookup or Lookup2. You can even change the slope of your ramp easily by adjusting the dwell time at each point, etc.

I do this sort of thing a lot for thermistor temperature tables, etc. Saves the time of executing the extensive formula for thermistor values.