Setting dropbox "out of"/ScoreDenominator from API

I'm trying to create an assignment dropbox and a new grade item from the API, with the dropbox linked to the new grade item.
Everything works in isolation, but when I add the AssociatedTool parameters to the call which creates the Grade object (to link it to the dropbox), it generates a 500 error.
Trial and error has shown that this is a result of the dropbox not having the "Out of" field set, which seems to translate to the ScoreDenominator value in the JSON block. This seems to be read only, if I try to set it the value I supply gets ignored and the result shows it as null.
I can't set that parameter either in the initial POST request to create the dropbox or using a PUT request to update it. Am I missing something, or is there a better way to do this?
Example input:
{
"CategoryId": null,
"Name": "(01-2) Computing assignment 1a",
"Assessment": {
"ScoreDenominator": 100.000000000,
"Rubrics": []
},
"CustomInstructions": {
"Content": "",
"Type": "Text/Html"
},
"Availability": null,
"GroupTypeId": null,
"DueDate": "2018-04-12T16:00:00.000Z",
"DisplayInCalendar": true,
"NotificationEmail": null,
"IsHidden": false
}
Example output:
{
"Id": 68,
"CategoryId": null,
"Name": "(01-2) Computing assignment 1a",
"CustomInstructions": {
"Text": "",
"Html": ""
},
"Attachments": [],
"TotalFiles": 0,
"UnreadFiles": 0,
"FlaggedFiles": 0,
"TotalUsers": 1,
"TotalUsersWithSubmissions": 0,
"TotalUsersWithFeedback": 0,
"Availability": null,
"Assessment": {
"ScoreDenominator": null,
"Rubrics": []
},
"GroupTypeId": null,
"DueDate": "2018-04-12T16:00:00.000Z",
"DisplayInCalendar": true
}
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