Confusion about how to compute probabilities for linked genes
0.000625 looks right to me.  On one chromosome, you have a 50% chance of a.  Then you have a 40% chance of b (if and only if a crossover occurs).  These multiply out to 20%.  You multiply that by 50% for three unlinked genes to get 2.5% abdef.
Your approach was correct overall, but you used a 60% chance of b, so your final answer was too high by (60%/40%)^2.  Don't let capital and small letters trick you - ab comes from a recombined chromosome.