70gr 3FG and a .490 PRB will certainly be effective on deer.
Yes, 5gr increment load adjustments are more preferred in my view as with 10gr increments you can skip right over a great accuracy combination.
Your rifle (Investarms Hawken Sporter), being a carbine version with a shorter barrel (21") and with a fast twist (1:24") is why you have a slightly reduced load data. Too heavy a charge with a PRB and that fast of rifling and you will strip the PRB over the rifling and your accuracy with go to pieces.
The bigger question is what accuracy will you get with 70gr 3FG and your rifle? Give it a try.
If when loading heavier hunting charges you are having accuracy issues, try a wonderwad over the powder charge and then load your PRB. Often with heavy charges, I find with the use of a wad and a tighter patch (.018 ticking) allows for the best accuracy/heaviest loads.
Shot placement is really everything but as mentioned by Rockypointjack, sometimes the optimal shot placement is not what you end up with for a variety of reasons. For that reason alone I insist on as much down range energy as I can muster for the give projectile choice.
For larger game hunting Deer on up, I always load up to the maximum that still holds decent accuracy while within the loading limits of the rifle for the selected projectile choice. Why? Because in a hunting situation, I never want to leave any down range energy on the table. Particularly with roundballs as they run out of gas much more rapidly as they travel down range. I load for 100 yards range with open sights.
With your rifle setup (21" barrel & 1:24" rifling) I would strongly suggest that you consider a ball-et or a light to medium conical or sabot. A PRB will certainly be effective, but I were carrying your rifle a field for deer the last choice I would use would be a PRB unless I were hunting close tight areas with a 50 yard or less range.
Try the Hornady Great Plains 385 gr hollowpoint or the T/C Maxi-Hunter 350grain. With 70gr 3FG and that conical or sabot in the 200-250grain, you will have a huge increase in downrange energy and likely a bit better accuracy with heavy charges down range.
Disclaimer: I realize that a sabot or conical is not traditional and I apologize for any heartburn caused by me mentioning them here. I think Jax12 is looking for a straight answer for his rifle and that's what I'm trying to give him.
:thumbsup: :imo: :m2c: