Hi there. I too make and sell. I have a refractometer just like yours and if the reading is 65% sugar then that is what I put on my jars. The 65% measurement tells you that in ever blob of jam or marmalade 65% of the blob is sugar.
I have been doing this since 2010 and no problems.
It seems like you are doing your selling the correct way, so well done.
it really gets me cross when I see others who obviously are not doing it the right way, and undercutting those of us who are.
Saw 2 people last Saturday, no ingredient labels, sticky jars, little cloth hats to hide the reused lids, and jars all different sizes and the front labels skewed.
Do you have public liability insurance? This can be very expensive, but the people I go with are the best priced I have found.
PM me if you would like the address and number.
Or if you would like to share experiences and support.
Good luck with your enterprise. I have several regular spots and it is great when you get regulars who come back week after week.