I’ll let Scott J. speak to the specifics of a ratio with ME, but I can comment on general proportions of intensity.
In order to maintain aerobic capacity in a long-duration event, I don’t think that you want the minutes of high-intensity training to exceed 5% of total training time. This’ll vary by individual, but that’s a rough guide.
(I say “minutes of high-intensity” to distinguish it from counting volume by sessions.)
I hope that helps.