A good place to begin is to add or subtract one-half kg for every 20 kg of body weight, plus one-half kg for the weight of your equipment.
Does this mean to add or subtract 0.5 kg for every 20 kg of body weight, and then add 0.5 kg?
Yes.
In this context, the word "plus" means "added to.. / and then add...", so the sentence means:
A good place to begin is to add or subtract one-half kg for every 20 kg of body weight, and then add one-half kg for the weight of your equipment.
±((0.5 × 𝑁/20) + 0.5)
, where 𝑁 is the total number of kilos). Hard to know if that makes sense in situ without the rest of the context though.