I'm lucky if I have a + balance.
Depends on how easy it is for you to deposit. I'm already assuming you trust the organization. For me
100 times my average bet or more if I have a good relationship and high level of trust.
Other than that the only reason to deposit somewhere else is to get a deposit bonus or join Matchbook for their lines.
A place I was new with, I may send in 10% of my overall bankroll and go from there.