Yes the small rule will do nicely, but you need to write the setting down. Spirit marker on the frame somewhere (or whiteboard marker if you can guarantee not to rub it off before you need to know it again). It changes with every different blade, which is why I use a tab of sticky tape.
The pivot arm, that the counterbalance spring attaches to on a 4x6, is only 60mm long from pivot to spring retainer bolt, so the spring is very strong.? Your head is heavier so maybe a spring from Harbor Freight's spare parts dept would be better than the one you've got.? You could slide the top mount of the spring inward toward the pivot point to reduce the tension in the spring and adjust bow weight.
You don't have to have an extension balance, like the luggage weigh scale, to find the bow weight.? If you've got kitchen (or maybe bathroom) scales you can rest the head of the saw frame casting on it.? Just have to arrange that the weight presses roughly in the middle of the scales (maybe a short stick?).
Looks like you've got it under control - jv
