The number after of is always the whole. Remember use CtrlF9 to insert field braces do not type them yourself. One option is to multiply the merge field with 100 in Excel and make the mergefield code look like this.
You can type in the Formula box select a number format from the Number Format list and paste in functions and bookmarks using the Paste Function and Paste Bookmark lists.
If the cell you selected is at the right end of a row of numbers Word proposes the formula SUM LEFT. Remember use CtrlF9 to insert field braces do not type them yourself. Word does not have a concept of percentages. On the Table Tools Layout tab click the Formula button.