How would I calculate a ratio based on specific criteria?

I am trying to automate the calculation of time required to create training, based on industry averages.

For example, it should take approximately 49 hours to write/design 1 hour of basic (Level 1) learning (49:1).  The problem is training isn't always a nice, clean 60 minutes, which brings me to this forum.

I have two inputs and one results field:

input: Level of Training (Level 1, 2, 3 or Instructor Lead)
input: Length of Training (in 15 minute increments)

:result: Estimated Design Time

The Level of Training determines my ratios: Level 1 49:1.  Level 2 79:1, etc.  The problem that I run into is when I enter a Length other than a whole number (.25, 1.5 etc) and try to calculate, it creates results that are less than favorable (not unlike my last relationship).

I've spent quite a few hours trying to research ratios (and how to calculate them) since I wanted to solve this on my own, but cannot seem to find a solution via that route (as you may have noticed, math is not my sweet spot).

Any help would be greatly appreciated!