First break down what the factory recommendation is:

5% of 29 gallons= 1.45 gal of pure 100%bio diesel allowed with each 27.55 gallons of pure 100% diesel.

So, @20% bio diesel, 1.45/.2 = 7.25 gallons of 20% would get you the max (1.45 gal) of bio and 5.8 of pure 100% diesel. Now, add another 21.75 gal of pure 100% diesel (27.55-5.8) and you should be at the recommended mixture rate.