Most of the secondary stats like Hit+, Crit+, Eva+, Protection+ convert to a percentage at 10 to 1, so simply divide by 10 (or multiply by 0.1) to get the actual percent. If it says 300 Hit+ then it will be 30% Hit.
However, the % version of these stats are a percentage of the actual stat that you obtain from base stats, not the + stats.
For example, if you had 1000 Hit+ on items and another item with 10% Hit, it's not 10% of that 1000 Hit+, it's 10% of the Hit+ you get from your base stats like Agility, Intelligence etc. So equipping an item with 10% will not actually give you a 10% improvement on your current Hit% rating that you see in the Hero window, it will usually be much less unless you have an enormous amount of Hit from a base stat.
Anyway all of this will change in the new year as I am reworking how secondary stats work and what you obtain from the base stats. I will most likely change how the % stats work also so that they actually give what they say, but it's still in the design/thought stage at the moment so won't give further information right now.
I do like the idea to have what you will gain/lose in the tooltip compare - I will keep it mind