Use yeastcalc instead since it's good with stepped starters. You'll find that generally, If it says 6L, you can do a two step starter and only use about 3L. IOW, a 1L followed by a 2L will get you close to just one big 6L. just a rule of thumb.
In the long term, you'll really need a bigger vessel. A $5 4L growler from the LHBS is pretty effective.