Gradebook

I think you were on the right track with the item multiplier. However you could accomplish the same using a multiplier on the category (if all 1.5 point assignments were organized into one category). 

Another option is switching aggregations. I'm going to make a couple assumptions here so correct me if any are wrong and I can adjust my recommendation. But first off, I believe you would have to edit core code in order to get the Assignment module to display decimals as an option for a grade. So that idea may not be feasible for you (and probably not a good idea anyway).

I'm assuming that you have all of these 1.5 assignments grouped in a category together and that no other assignments of different point values are included in this category. I'm also assuming that the number of assignments is known and fixed. If both are true, then switch this assignment category to Simple Weighted. What this will do is allow you to enter a point value in the category total that the total of the scores will scale to. You would still need to create the individual assignments as 15 points and put them in this category though. For the category total, enter the total the assignments should add up to (number of assignments X 1.5). What the gradebook will do is get a total of the points earned, divide by the points possible and then scale to the number you entered in the category total. See below for an example.

Two key notes before the example though:

• I recommend turning OFF the Aggregate only non-empty cells setting for the category in the Full View of the Categories and Items. With this setting enabled, and if the instructor has entered all their assignments beforehand but not ended grades yet for all assignments, it will give students a grade for only assignments they have received grades for thus far. Cool!... but also has it's downsides. If a student only turns in one assignment and receives a 100% on it, the category total will show as 100% because the category total is ignoring the empty cells. The teacher would need to enter a zero for all the assignments the student didn't do. I recommend turning it off because if your teachers are like mine, they forget to enter zeros. So you lose the benefit of showing an accurate grade at times but at least that way you make sure the final grade (which is most important) is accurate.
• If you end up adding in another assignment after you already entered the category total, make sure to adjust that as well. It won't adjust automatically like the Sum of grades aggregation does.

Categories and Items example

Grader Report Example

Happy oodling!