Gambling Your Fifth edition Character's Life

I was flipping a coin mixed in with my RPG dice and started thinking about a fun way to use a fifty/fifty probability dynamic in a game. It seemed like a basic gamble and reminded me of a simplified dice game. Gambling of coarse is always the most exciting when the stakes are high and a save or die situation came to mind as something exciting to try. 

D&D currently has the death saving throw - a series of saving throws with a target number of 10 rolled on a d20 without modifiers. Its basically a race to see what outcome is rolled three times first - 10 or over (save) or under ten (fail). 

Including the ten in the target number adds five percent.

I started thinking that I could substitute the coin flip for the d20 roll as the target number is half way and essentially an over under situation, but thinking about it further I realized that its not quite what it seems. This probably isn't a revelation to anybody that thinks about how dice and probability work but a roll of ten or over isn't a fifty percent chance either way, its a fifty-five percent chance of making a successful save. If each digit represents five percent then failure to make a death save occurs only forty-five percent of the time as 1-9 represents the range of failure. It sounds fifty/fifty but that would actually be if success was a roll of over ten.

Rolling a one counts as two failures!

So I guess if you wanted to use a coin flip to represent a death saving throw your odds would be less and that would be a real gamble.