Well, it would have been more useful if you hadn’t turned on the concentration button. By pressing that, it shows the skill as 284, which is the boundary line for R4 (355 difficulty * 0.8 = 284). It would have been better if you had not pressed it so we could check your actual skill. I’m not going to waste my time working backwards from your concentration value to get your actual skill.
I concur with @Solaria and @Kaladnei. Your addon is not correct. If you put R3 mats you will get an R5 result.
R1 mats give 0 skill points. Mats are 40% of the base difficulty, which is 142.
The boundary line for R4 is at 80% of the overall difficulty, so you absolutely cannot get to R4 (without concentration) without at least R2 mats (R2 mats count as 20% of the base difficulty). If you have no embellishments, missives, or crests, placing R2 mats should put you right at this line. With R1 mats, your result will be R3, as the base UI is showing correctly.