The spell bonuses are all on the relevant wiki pages, for example:
https://en.touhouwiki.net/wiki/Mountain_of_Faith/Gameplay#Spell_Card_BonusTo summarize:
EoSD spell bonuses are based directly on remaining time, with a multiplier that varies by stage.
PCB/IN initial spell bonuses are based on the specific spell. They decrease particularly quickly compared to later games, but can be increased by grazing (PCB) or collecting time orbs (IN).
MoF initial spell bonuses are based on stage number, and current faith as of when the spell starts.
SA+ initial spell bonuses (including GFW) are based on stage number and difficulty, with a multiplier that varies by game (the multiplier is too low to be significant in SA/GFW/TD/LoLK though).
ISC initial spell bonuses are based on day number only, and are awarded even on timeouts.
Most spell bonuses decrease over time, though in MoF+ the bonus stays constant for the first 5 seconds, probably so you can read it. Survival spells' bonuses never decrease, except in EoSD (where Flandre's is always worth only 700,000 points), and ISC 3-7 (where you can still capture the spell but Mokou just re-declares it until time runs out).
Easy-Lunatic are difficulties 0-3 respectively. Extra is difficulty 4, and stage 7 in most games (stage 1 in GFW). TD Overdrive is difficulty 5.
You should also keep in mind that spell card bonuses should be made significant enough to not simply ignore when scoring your game, but also not too significant such that spell card bonus is one of the only things that matter.
This is particularly important though.
Also if you don't score but you're trying to create an interesting scoring system, it'd probably be a good idea to have someone with scoring experience help out, so that the scoring system doesn't turn out very different from intended.
For example, I doubt ZUN intended for TD scoring to be about bombing/trancing nearly all the boss phases (the patterns seem designed around risky point-blank shooting). Also I'm pretty sure Reisen was supposed to be the best at grazing in LoLK, but Sanae bomb-grazing ended up more effective on higher difficulties.