Another approach could be to "pre-steep the alcohol away". Mix (50-50 - ? - not real sure, experiment around) of the "pure chocolate extract" with some plain pg. Leave this concoction out overnight in a small flat bowl. (Maybe longer -??)
Take 20 drops of your "base" and try mixing 1 drop of this chocolate for a 5% mixture, 2 drops for 10%, and maybe 3 drops for 15%. (Actually it is 19 and 1, 18 and 2, 17 and 3 mixture for the exact percentages...) (OK, Yea, I know...drops and the word exact do not really belong together...)
That's what I would do... After all, a large part of DIY is the experimenting...
Good luck.