I have to be honest, I just assumed that they were going to go along with their 'equipment level' basis of if you are wearing rank 15 (lvl 114) gear then you get 15 * 5 Blue magic skill extra added into the equation for calculating damage. Capping of course at +100 skill on Delve Mega Boss gear.
Going with 5 as a fairly average skill increase per level, but of course it can be between 3 and 7 historically, or if it's still under-powered, make up their own new amounts, or even have them scaling differently for different levels. Maybe +10 for the first 5 levels, then +7 for the next 5, +5 for the next and +3 for the last 5.
That way at least ALL lvl 100+ gear would allow people to progress as long as 1 was equipped during casting, not just specific pieces that have magic damage+ on them.