i messed up my roth ira

for 2025, i contributed 7k in a traditional and 7k in a roth (i didnt know you can only have 7k in one). so now im in the process of removing excess. im making more than the roth limit, so ill have to remove my current roth 7k, and then convert the 7k in my traditional to roth.

couple questions:

when i do this on schwab, it is asking to specify how much tax witholding percent. (default 10%). what does this mean and what should i set it to given my situation?

and also, what should i do in this process to make sure i dont mess up and dont pay more in penalties than i have to?