Ideally, one would measure the chord (leading edge to trailing edge measurement) at a given point, in the case of the Sparrowhawk right at the fuselage, and then measure the balance point. That point would be expressed as a percentage. In most cases, the percentages fall somewhere between 25 to 35 percent of the chord.

The parts on the tip you referred to are called tip gussets.

I wouldn't trust the spar as the assumed balance point as some designers can put them pretty far back. The thickest point of the rib profile, sometimes called the high camber line, is usually a fair guess however.