I have mixed views on carbon offsetting but for example purposes – based on recent travel.
Aeroflot have a useful carbon calculator (in English as well): https://m.aeroflot.ru/ru-en/co2/calculator
So return flights from London to Irkutsk are 1008.78 kg / 1.01 tonnes. Carbon Footprint.com suggests some reforestation and clean energy projects for 1.01 tonnes ranging from approx £6 – £25 but if I chose to offset by a certain % of flight cost, I would prefer to donate to scientific and community projects in the geographical atmosphere I’m polluting and also into research into alternative fuel for aircraft / alternative ways of flying e.g. https://www.newscientist.com/article/2181821-how-to-make-jet-fuel-from-used-cooking-oil/