No. WDH kits have a very wide range in TW and still be effective. I use a 10k Equalizer for about 750 lbs TW, and I hear you can go lower than that.
Size the WDH kit for the maximum condition, then make adjustments if the "effective" TW will be significantly less.
As far as how to weigh the stuff behind the rear axle, we're talking about what cargo you put in the bed, behind the axle. Just estimate it.
thebrakeman ('70), DW ('71), DD ('99), DD ('01), DD ('05)
2004 Surveyor SV261T (UltraLite Bunkhouse Hybrid)
2006 Mercury Mountaineer V8 AWD Premier
Equal-i-zer WDH (10k), Prodigy Brake Controller