>>>Should be simple but I'm rusty..
>>>
>>>In a Form.Init() I create a cursor:

CREATE CURSOR xxx (
>>>    MyField N(6,1)
>>>)
>>>APPE BLANK

If I open this in a browse window I can enter data as expected (e.g. typing '12345' gives '1234.5').
>>>In the form I have a textbox with DataSource = xxx.MyField.
>>>If I enter the same keystrokes here I get '1.24' and it tabs to the next item.
>>>
>>>What am I missing?
>>
>>It should be ControlSource, but I assume this is a typo.
>
>Yup, sorry - typo.
>
>>Check what is InputMask property of this TextBox.
>
>I'm not setting it in the designer and it's an empty string at runtime....

Try to set Value of this TextBox to 0.0 before settinmg the ControlSource:

thisform.TextBox.Value = 0.0
thisform.TextBox.ControlSource = "xxx.MyField"