Im unsure how to calculate it myself, but if you download the trial version of the staking machine, you can generate up to 4000 samples using your SR and av divis.

I just ran a test of 4000 with your SR, and a longest loosing run of 21 came up in the frist sample. Most other 4000 run samples were around teh 16-17 mark, and the worst I could get was 23. Thats where a staking plan can run into big trouble.