Hi Navdeep,
I am not sure if you are missing out on this info. When you create the portlet from the object/sub-object, then within the 'Fields' sections, you can click on that particular attribute, which in your case is 'Allocation Percentage' and scroll down within that Properties Page, and mark the 'Allow Grid Editing' field to be true. So, when you click on that particular investment link on the portlet within the application side, you should be able to make changes to this attribute. Hope this helps.
Suhail.