Is there a way to find out how big will be the faction loss for the other guy if my standing is e.g. 700 and he has only 200 points?

You just learn it. Standard hit for a subfaction is around 200 points and standard for a major faction is like 600-800ish so if you're POS with a major faction it generally drops you down to neutral.

The faction hits should be considered in context with the difficulty it takes to restore them. You can generally do 2-3 proc missions to restore a kill in a subfaction and a few more in a major faction. If you're planning on participating in regular murder around stations then stock up with the common item for that faction:

Itani: XiRite Alloy to Jallik E15
UIT: Organic Solvents to Verasi O7
Serco: Ion Cores to Initros O12
XX/TPG/Orion: Consumer Robots to Latos
Tunguska: Phase Arrays to Edras
Corvus: Consumer robots + phase arrays to odia and bractus
Ineubis: Phase arrays to bractus

Don't be afraid to tank your standing because there is a clear path to faction restoration with any faction now. With the major factions you can undergo botting to restore to dislike then undertake proc missions. With the subfactions you can take diplomatic TRIPS which restore to dislike for 20,000,000c once per week per faction; Dau Senate for UIT subfactions and Corvus Capital for grey factions.

Happy gratuitous murdering!

Thanks for your answer!

Consumer robotics will also work at the Ineubis and Tunguska stations in Odia. Meaning they work for every grey faction but Aeolus. Just maintain stockpiles in Latos and Odia and apply as needed.

very valuable info above!