[nigelH]

I don't like handing out formulas but I'll explain it.

Get the pressure at depth so for 100m this is 11 bar (call this depth in bar)

Subtract your oxygen setpoint eg: 1.3 giving 9.7 bar of inerts
Get your inerts in the DIL 10/70 = 10/70/20 so He:N2 = 7:2
Split the inerts the same way so He = 9.7/(7+2)*7 = 7.5bar
Whats left is N2 so 9.7-7.5 = 2.2bar

You are breathing 1.3/7.5/2.2

We can work anything out with these:
END = depth we are breathing that much nitrogen on air
depth in bar = 2.2/0.79 = 2.8bar so 18 meters EAD
EAD (not a really well defined term for 'mix but lets say it's total inerts) so 9.7/0.79=12.3bar so 113m (deeper because you are breathing less oxygen% than air.

Most diving calculations are simple one liners if you work in 'depth in bar'. It removes all the multiply/divide by 10 and do I add or subtract 10 stuff because you can always remember how to do depth->bar->depth in your head.

(edit) Forgot the density bit... forget my head next...

We need some constants
O2 = 1.429 grams per litre at 1 bar
N2 = 1.250
He = 0.179
Air = 1.288

so 1.3/7.5/2.2 becomes 1.3*1.429 + 7.5*0.179 + 2.2*1.250 = 5.95 grams/litre
so convert back to air is 5.95/1.288 = 4.6 bar
so depth is 4.6 bar = 36m

(/edit)