When determining map distance between two genes that are far apart on the same chromosome, it is most accurate to add up the recombination frequencies of genes between the two outside genes because
The recombination frequency between the two outside genes will be an over-estimate of the actual distance due to double cross-overs
By adding up the smaller distances you can account for MANY of the double cross-overs that occurred between the two closest genes.
By adding up the smaller distances you can account for MANY of the double cross-overs that occurred between the two outside genes.
By adding up the smaller distances you can account for some of the parentals, which will look like recombinants when using the two outside genes.