Avoiding "hot roots" coloring gray hair help needed!

I'm trying to help my wife and we're looking for advice!  Some background: She is 50 and has a lot of gray. her hair has been previously colored and is a medium brown. She wanted to go more reddish brown/ mahogany.  She used Madison Reed Trieste Red https://www.madison-reed.com/product/trieste-red 10 days ago.  All in all it looks great- a really nice brownish red.

However, after the application of the Trieste she ended up with "hot" roots - about 2" of brighter red, when compared to the lengths. however, in about a week it faded so the hot roots are much less noticeable and is something she can live with. we both really love the color and  wouldn't mind her hair even ended up being a bit redder. i have a few questions for the experts:
--how can we avoid hot roots next time?
--the colorist at Madison Reed provided the following advice for next time: either color only the roots and let them process for 45 minutes. OR if we want to refresh the overall color, we would do the roots, wait 30 minutes and then do the rest of the hair and wait for another 15 minutes, adding that this would make her hair redder and it would also further the look of the hot roots.

I've done some research and have read about various suggestions: doing a deep conditioning prior; being careful to not putting the color too close to the scalp because of the heat generated from the scalp; doing the roots first to allow more time to process and doing the roots last for less time to process! We really need some clear cut advice to continue using this great color but also avoid or at least minimize the hot roots - and if a bit redder hair is a byproduct of the suggestions, then that's ok too!  thank you in advance!