When adding ETF Holdings to the Symbol Lists the WEIGHT() function can be used to show the percentage weight that a stock has in the ETF.
Once the members of the ETF has been added to your symbol list you can show the percentage weighting in a watchlist column using the Fund Weight column. However, as a stock can be in more than one ETF the Fund Weight column will only show the weight of the last ETF list opened.
To show the weight of a specific ETF use the WEIGHT() function to a new custom column and click on the text to select the required ETF from the Symbol List dropdown (remember, the ETF must have been added to the ETF Holdings list first).
For example, this will show the weight of each stock on the Nasdaq 100 ETF ($QQQ):
WEIGHT(SYMBOLLIST=QQQ - Invesco QQQ Trust)
In the watchlist right-click on the column header and change the Column Type to Percentage, and add a Column Label. This example shows the weights of $QQQ and the SPDR Technology Sector ($XLK) in one watchlist: