Pulse Oximeter signal processing
Unless you're near dead, your \$O_2\$ sats are not 86%.  Getting pulse out of a pulse-ox signal is dead easy, getting \$O_2\$ out is not.  In fact, while playing around with a Maxim pulse-ox unit, I couldn't even get Maxim to provide a formula unless I signed a non-disclosure with a waiver of liability.  

This is fairly hard to calibrate.  I'd say assume your level is 99%, but how to get the other point on the low end without a full lab and blood gas testing is beyond me.

You might consider actually calculating the rms values instead of approximating.