You've put the cart before the horse .
Firstly , regardless of how a case is sized , a reduction in c.o.l will cause a reduction in case capacity and an increase in pressure , not a decrease .
If you're trigger is so heavy as to affect your ability to shoot accurately, how can you trust your results . I suggest you sort the trigger then start again.
I'm not sure why , but you appear to be in an awful hurry . You say you're new to reloading and then have jumped in feet first .
If you're intent on doing seating tests , find the lands first , modern thinking suggests seating 0.020 " from the lands when doing initial development of charge weights . Though , magazine length may be a limiting factor as may bullet length if there is to be sufficient in the case neck . Then after charge weight tests do your seating tests .
My suggestion would be to use the bullet manufacturers recommended c.o.l .
You may struggle to get out to 800 metres as it's doubtful that a 16" barrel will give sufficient velocity for the bullet to remain super sonic at that distance .
Good luck , I hope you achieve the results you're looking for .