With majirel you mix 1/2 base in when there is over 50% white hair.
If it is a cool tone mahogany
i.e. ,52 (Mahogany-Violet) / ,51 (Mahogany-Ash) mix with a natural base
If it is a warm mahogany
i.e. ,56 (Mahogany-Red) / 54 , (Mahogany-Copper) / ,35 (Gold-Mahogany) etc.. mix with a golden base (,3)
You also have majirouge with give you more vibrant results but the same rules apply for white hair coverage.
You then have a few shades marked DM5 (Red-Red Violet) and Rubilane (Red-Copper Red) which give 100% coverage without adding base. However if the lengths and ends have more colour build up/ depth the regrowth will still be more vibrant. And you shouldn't add base with these colours.
I would mix your target shade with base to get the white hair coverage, then after rinsing if the colour is not vibrant enough, run a mahogany-red dialight or diarichesse glossing over it for 5-10 minutes to kick it up a notch. Likewise if the colour ever goes too pinky apply a glossing in a natural gold shade. Hopefully you won't need this step, but it's there if needed!