Maybe check that your back plate is perfectly flat to begin with, from your pic it looks like you don't have bolts in all the holes to ensure it is mounted flat and firm. I made that mistake with mine and had a 3/16 gap become less when I mounted the propeller and did all the bolts up, I had to do some reworking to regain the gap.
If your pink cowl is anything like mine was (ex Van's) there will be some fitting, trimming and filling required.
RV 3B. Very slow build. Now learning about paint..