I need to drill a 5/8" diameter hole through a laminated beam that is 16"
deep. This beam is sitting 10 feet above the floor across a room support
part of the roof. So I will be standing on a bench drilling from below
pushing the drill up.

I need this hole to be perfectly vertical as I will be passing a rod through
for a ceiling fan.

I know in the past when I had to drill through something much thinner like a
2x4, I often end up with a slanted hole.

What can I do to ensure my drill is perfectly vertical?

I cannot afford to make a mistake and redrill.

Thanks,

MC