Why do you need so high regression? It's impossible taking into account all sources of experimental errors. I would be happy to have R > 0.98. In addition, you have abs < 0.027, which is too low for an instrument.

I am no expert in spectroscopy, but Conncentration as a function of absorbance, for a specific wavelength should contain the (0,0) pair of points, right ? When you don't have any compound that gives signal at that wavelength, the concnetration should be 0. Yurii V Geletii is kinda right. A regression coefficient R^2 ~ 0.9999 is something that might be : 1) unachievable, 2) economically not advantageous, depending on your application.

Regression values are related to measured data and can only be improved through more careful designing and generation of data. I don't know how sensitive your instrument is; regardless, as commented by Gelatii, your absorbance range are very low! Generally, best results are obtained at about A = 0.5 that would mean increasing calibration concentrations ba about a factor of 20-50. Hope then you will not have same basorbance values (A=0.021) for different concentrations (6 and 8 ppm); also, you may get better regression value.

Oh thank you so much sir for your valuable time 🙏 glad to see your response towards me thank you so much 🙏