What is typically the best bolt to hold Park BBT-22 on to square-taper spindle? If one is trying to remove a stuck square-taper bottom bracket, it is hard to both hold the tool onto the bottom bracket splines and turn it at the same time. Many tools, like the Park BBT-22, have a hole so you can bolt it down using the crank bolt hole in the spindle, so you can just focus on turning it, or so you can just hold the tool in a vice and use the bike frame as a lever. 

However, documentation seems fairly sparse about exactly which bolt one needs for this. Obviously it's like a crank bolt, but longer. But what's crank bolt threading? What length works best? This forum thread has the advice "Get an M8x1.0 bolt 40 or 50 mm long. Note the thread pitch is 1.0 mm, not the more common 1.25 mm that is usual on M8 bolts." Can someone confirm that is correct? It seems hard to mess up the threading, but the answer is from thirteen years ago, refers to a BBT-2 rather than BBT-22 (typo?), and it's not clear what length is optimal. Hoping someone has such a bolt in their tool drawer and can just measure, so I can order the best bolt (and I'm sure I won't be the last person to have this question).
There’s actually a tool made specifically for this purpose. For example, here’s a link to the Pedro’s version: 

https://pedros.com/products/tools/pedal-crank-and-bottom-bracket/bb-socket-holder-ii/

They call it a “bottom bracket socket holder” so try that as a search term for others.