At the beginning of my contracting career I neglected to think about all the time I'd spend on non-coding tasks. It wasn't insignificant. I also found that the 4x full-time hourly wage is about right. A few other things I learned:

Almost never take a fixed bid contract. They're pretty much always a net negative.

Give clients discounts for paying early. Everyone loves a discount. Never discount your hourly rate though, because once you do that once for a client they'll never stop arguing with you about it. There's a line on Schedule C for recording discounts.

If you turn over raw source code to a client, stress (in written, contractual form) that if any other contractor/employee changes the code, then you will charge them a premium rate for any changes or fixes.