Drop down list set to default sum but blank when row is blank

Hello. I hope someone can help me.

I have rows of six columns, the forth of which is a drop down of percentages. I have chosen one of those percentages to be default (66%). However, I don't want that 66% to be visible unless there is something in another cell. Example:

Column B is a Materials List
Column C is Quantity
Column F is Markup and has data validation set to a list with markups from 25% to 75%. I want 66% to appear every time a material is added to Column B. Then if the markup is less or greater, the drop down can be used.

Any ideas? Thanks!

Argh, your sheet works great but I can't get it to work on mine. I get "Value not valid, restricted values..." error.

Edited to add: It's .xlsm file.