Well I just did some rough playing around with the spreadsheets. Since we are normally looking at a fixed end mix and gallons, I broke it down to ratios of the total.
For 16Gallons of E30, you need 4gal of E85 and 11gal of E10. So when mixing E10 and E85, to get E30 you need 26.6666667% of the total gallons to be E85 and the rest is E10.
- So for 7.5gal of E30 it is: 7.5*0.266666667=2gallons of E85. 7.5gal-2gal=5.5gal of E10
To make E20 based on the same total of 15gal it takes 2 Gal of E85 and 13gal of E10. So the ratio of E85 is 13.3333333477778% of the total volume.
- So for 7.5gal of E20 it is: 7.5*0.133333333477778=1gallons of E85. 7.5gal-1gal=6.5gal of E10
This is far from perfect way to do it. If you do not have E85 that is actually 85% (falling short and is actually 83, etc) then this will not be correct. That is why I was hoping someone could work up a formula to do it on the fly to account for variances.