If you are using 1.9.5, the simplest way to do this would be to manually add a "grade item" to your Gradebook. If you are not using 1.9.5 you can still try this because I am not sure which version first offered this option.
Course Admin menu > Grades > Categories and items > Simple view
Scroll to the bottom of the page and select the "Add a grade item" button.
Enter the data required - the "Item name" shows up in the gradebook.
Grade type = Value
Maximum grade = 150
When you are ready to enter the grades this will have to be done directly in the Gradebook. It would be a good idea to turn on "quick grading" and "show quick feedback" in your gradebook "my preferences" page.
Course administration menu > Grades > My preferences > Grader report > Scroll to the bottom of the page and select yes for both options.
Follow these steps when you are ready to enter the grades.
Course administration menu > Grades > View > Grader report > Turn editing on
After you turn editing on, you will see blank boxes where you can enter the grade directly. If you choose to enter grades like this be sure to click the "Update" button at the bottom of the page to save the grades. You can also click the edit icon, which will open a more detailed form which allows you to enter the grade and more data if needed. There is a "save changes" button at the bottom of each of these forms.
If you choose to create a custom scale for grade items like this you will be required to create a new one every time the total number of possible points is different. It is much quicker to to manually create a grade item and just type in the total possible points.
One disadvantage of using this method is that the grade item will not show up on the course page like the "Offline Assignment" type will. However, it does show up where it counts - in the student's grader report and in your gradebook.
If you choose to create a custom scale for 150 possible points you will need to be sure that your site administrator has enabled the setting on the front page for "Include scales in aggregation." If this setting is enabled, it may also cause problems across your entire site because some teachers may not want to include scales in the aggregation of grades for their course. Moodle translates all custom scales into a numeric value before aggregating that grade and this will result in some unexpected final grade averages in courses that use a lot of custom scales.
Please read this info from Moodle Docs
before you choose to use a custom scale.