This scares me. 3.5 grains less than what listing? Max or starting load?
If below the Max listing,. I would hope so. If below the starting load, did you ever hear of detonation?
Fifty years and 100 pounds ago I was a skinny kid just starting to reload. I was loading for a British 303 and determined that I wanted to load some light loads. The first one fired detonated and froze the bolt. Eventually I got the bolt to open and the case head had flowed back into the bolt! I was lucky. I still have and shoot that rifle. Get the point?
As for getting better accuracy by adjusting your load. That is the name of the game. After selecting you bullet and powder, load a sample (I normally go with 5 rounds) within the bracket of minimum and maximum as listed in a loading book that I trust (I will compare several book's data.) Depending upon the amount of spread, I will load a number of samples. This is with fully processed and consistent brass and the same primer type. Then test fire each group in order, looking for any pressure signs as I go. As each test group is fired at different paper targets, I will get a good idea where the 'sweet spot' is going to be for that weapon. There is no rime or reason to which loading combination will work better or worse in a given weapon. The results are some powder charges will work better or worse. I load for two different M1 Garands and they don't like the best loading for the other.
The good news is the loading for the SPFD is so close to the preferred loading for my 03A3.
Things to be picky about
: trim length of brass (bottle neck cases stretch fast) - over all length of the loaded round (bullet seating depth). These can cause extreme pressure changes fast!
Sorry about the long post, my 4 year old grand daughter doesn't care.
Always error on the side of safety.