You want the probe to be in the center (as close as reasonable)... so the 5/8 measurement should still apply.
When I drilled mine, I used a 1/8 to start, and did not drill thru.
Then I used the tap drill, and stopped just short of breaking thru.
Then I greased up the tap drill and went thru, hoping to catch most of the chips in the grease. Then I used a pocket extendable magnet to get as many chips as I could... then a shop vac with small dia hose rigged to it.
Then I tapped... using grease on the tap... and did magnet and shop vac process afterward.
Hope this helps.
It's really a piece of cake.
Good luck.
Jay