Variance

[walker77]

I started a betting approach 6 months ago and over 2700 plays made around 130 units, about 5% ROI. Thought this looked pretty solid so stepped it up.

With a bankroll of 200 units, started the year with a strategy of betting between 0.5%-3% of roll depending on the size of the overlay and limits. After the first 400 plays, dropped 60 units, -12% ROI. I know I should know this but can someone calculate the % chance of losing 12% after 400 plays with a 5% edge?

I know it's too early to get scared given the previous performance, but would like to know the odds of downswinging like this. Things are always changing in this game so it's good to be vigilant and able to identify when what you were doing previously may no longer be +ev, without thousands of samples to confirm.