I'm finally happy with this color after doing it once a month for 4 months. It gets darker each time, and the gray is no longer a problem. This is what I used.

100g of henna in a mildly acidic liquid mix. After dye release, I applied the henna to my hair, but I only keep it on for about 3 hours; more than that made my hair too bright a red for my taste. I use a plastic cap with a wool ski hat over everything to keep in the warmth.

I'm going to continue this every month so the gray never reappears.I hope this helps some people who are looking for ideas.

Notes and References
Henna: https://www.mehandi.com/Mehandi-com-Ancient-Sunrise-Henna-for-Hair-s/121.htm
Don’t have fruit juice on hand? Keep some fruit acid powders in your cupboard.
Mildly acidic liquid: https://www.mehandi.com/Fruit-Acid-for-henna-dye-release-s/299.htm
More info on mildly acidic mixes: https://www.tapdancinglizard.com/henna-and-acidic-mixes/
Ancient Sunrise henna for hair kits with everything included: https://www.mehandi.com/Mehandi-com-Ancient-Sunrise-Henna-for-Hair-Kits-s/124.htm
Amla: https://www.mehandi.com/Ancient-Sunrise-Amla-Powder-p/aml_01.htm
Henna time and temperature: https://www.tapdancinglizard.com/mixing-and-testing-your-henna-mix pp. 10-13
Henna color oxidizing ("rusting"): https://www.tapdancinglizard.com/mixing-and-testing-your-henna-mix/ pp.25-29
Chemical hair dye allergy: https://www.tapdancinglizard.com/henna-for-hair-2/ pp. 18-26

Video How-To
Going red with henna: https://vimeo.com/manage/videos/739464950
How to mix Ancient Sunrise Henna: https://vimeo.com/manage/videos/770522457a>
Henna Root Touch Up: https://vimeo.com/manage/videos/767016457