NO NO NO!
STOP RIGHT THERE!
In a database if you are holding percents keep it as an integer or real. Not as a varchar, although php will convert it for you. BUT never store the percent sign, it goes against ALL database rules!
store the percents as numbers and add a % sign when you are outputting the text. Then your original query with >= will work.
Although if you cant or dont want to make an import script, you can trim and convert
I dont remember the exact syntax but this should be somewhat it:
toint(substr(successPct,0,2))
what it does is take the first two letter and convert them to numbers
letters:'12%' to int 12