Hi @ematsen,
if your tree is binary this is trivial, as all the necessary functions exist:

```
library(ape)
library(phangorn)
library(magrittr)
# a little function for you
get_l <- function(tree, dm){
# make sure the w and d will be in the same order
label <- tree$tip.label
dm <- as.matrix(dm)[label, label]
dm <- dm[lower.tri(dm)]
tmp <- designTree(tree)
w <- 2^(rowSums(tmp)-2)
sum(dm / w)
}
# a little example
data(yeast)
dm <- dist.ml(yeast)
fastme_tree <- fastme.bal(dm)
get_l(fastme_tree, dm)
# compare with brute force
trees <- allTrees(8, tip.label = names(yeast)) # compute all possible trees
trees %<>% .uncompressTipLabel() %>% unclass # this speeds up things a bit
bf_l <- sapply(trees, get_l, dm=dm)
bf_tree <- trees[[which.min(bf_l)]]
RF.dist(bf_tree, fastme_tree)
```

I have no code for the general case with multifurcations (yet).
Hope this gets you started.

Cheers,
Klaus