[ic] add fixed amount to shipping based on cart's total weight

[DB]

I currently use cost-based shipping calculation. I need a way to add an
additional, fixed amount to an order's shipping if the total weight of
items in the cart exceeds a certain value.

Can someone suggest how I might code this into my checkout page? Perhaps
something like:

  [if type=explicit compare="[calc][weight noformat=1] >25[/calc]"]
  [assign shipping=shipping+10]
  [/if]

I'm pretty sure that "assign" line lacks the correct syntax?

Thanks!

DB