It should be good enough, even though you are using a rue of thumb. You might try a screw extractor.
Here's the instructions on using a screw extractor.
Here's a link from a previous post on suggestions on removing damaged screws.
If you extract it in a good way you won't damage anything and can reuse the old parts. The screw extractor kit is handy to have around, even if it is only used a couple times a year, if that much. It easily gets the bad screw out, get a replacement handle and screw a new set screw back in. If you drill it out, the you risk damaging the threads or you might have to tap the hole and go with a large size that might stick out more than the designed screw.