The risk (entry-to-stop) alone should determine your trade size for any given trade. The amount you choose to risk per trade is up to you, but I would suggest keeping it the same for all trades and not exceeding 1%.
As for decreasing R:R nothing wrong with that if you're getting a higher W:L.
No magic in any of those forumlas, which is probably why they're useful.
In other words, study and practice until you become proficient at picking winning entries and limit your losses as much as possible. You must keep your mistakes as CHEAP as possible, everything else is peripheral. Some people will make you think this means using a slide rule and vernier calipers to calculate where your stop should be. These things will develop naturally as you progress. Anyone who has gone the DMA route and started with a limited amount of capital will understand exactly what I mean.