You can essentially have a boon that is 10%/ 20%/ 25%/ 35%/ 50%/ 60% or 100%.
The problem is the big gap between 60% and 100%. This becomes an issue with,
for example, the boon improved skills. If I as a demon have 88% skillmax, I
can boon it up to 98% with a medium boon improved by a bane, or 104% with a
full boon. I know you can take secret society levels to make up for the last
2%, but a fix to the boon system would give us a higher precision for
adjusting our boons.

Here's what I suggest: introduce a "large boon" that gives you 75% of the boon
(or 85% when improved with a bane). The (new) cost for boons could look
something like this:

Tiny Little Medium Large Full
Efficiency 10% 25% 50% 75% 100%
Improved with bane 20% 35% 60% 85% still 100%
Base cost to pray (TPs) 10 20 40 40 40
Total TPs (1st boon) 10 30 70 110 150

CM