I’ve been summoned! 
Co-Head of the United Standings Improvement Agency [USIA] here.
CCP publishes their standing gains formulas on their Standings support article on their website. I’ve already written a calculator for this for use with USIA: -9.48 (est.) Base w/ Diplomacy L5 = -5.584. (Note EVE actually tracks standings to four decimal places, so the value you see in-game is rounded and the final value may be slightly off by a hair.)
There are a few options for boosting your faction standings. The main methods:
- Hit up COSMOS/Datacenter agents (reference The Plan by DMC).
- Blitz L1 missions to rapidly generate storyline missions, each storyline mission would boost your faction standing. One of the better ways would be to take an exploration frigate (T1 or Astero is fine), give it a combination of cargo expander + agility mods/rigs, and have it do two (possibly three) L1 distribution missions at a time (find an Amarr faction station containing two or more L1 distribution agents). Yes, such stations exist outside of Amarr space if you don’t want to be harassed by facpo.
You only need to rise above -5 to stop being harassed by facpo. If you rise above -2, you’ll have access to the epic arcs (which give you nice faction boosts and rewards) + higher level missions (with their higher level storylines) as well. Also note that it is in fact possible to have positive standings with all factions simultaneously: USIA has written a guide on how to accomplish that (I have more content to add to the guide and could probably break it down into smaller guides, but there’s a lot in there already). If you have any additional questions or would like any more advice and guidance (we don’t charge for that 
), pay us a visit on the USIA Discord server and have a chat with us.