My 18" Tubliss rear setup needed five ounces of weight to balance.
The 21" front needed five and one half ounces.

Getting that many weights to stay stuck on the skinny 21" front rim
proved difficult. The rim cross section allows very little contact
with the adhesive on the weight, so I used double-sided foam tape as
a filler. With a little fussing and fitting it worked well enough, but
something better would be very desirable. Hot-melt glue seems like one
option, RTV silicone another. Both present removal problems, however.
The foam tape can be removed with solvent fairly easily.

In a couple of spots the foam tape wouldn't hold on its own. Use of
Weldwood Contact Cement has largely solved the problem. Just paint it
over the pressure sensitive adhesive. 

Five ounces is a substantial imbalance. It might not be an issue in dirt,
but any smooth surface at speeds over about 25 mph will be vastly improved
by balancing.