Can anyone offer some insight in to what concentrates would be a good starting point to clone this sinus cleanser please?
https://rejuiced.com/menthol-molotov-cocktail
https://rejuiced.com/menthol-molotov-cocktail
I've never tried that juice @oHUTCHYo, but going from the description and reviews I'd say try making a couple of testers with just menthol, one at the recommended percentage and one using double that to start with.
Taste them, and if one's too weak and the other's too strong mix them in various ratios (keeping notes of course) until the menthol seems right. Obviously if they're both too weak then try with a higher ratio until it's right. Then, if it needs it, add mint and/or eucalyptus and possibly sweetener until you get it just right, but only change one thing at a time!
I haven't looked, but there may be some clones available on elr e-liquid-recipes.com
Good luck!