.458 bulging Starline

[Kurt]

Doug.

Your overall seating length depends on what your bullet profile will allow. Like if you have one or two front driving bands reduced or the length of the ogive compared to the shank.

I wouldn't worry about an extra 3 or 6 grains of powder it will not give you that much for velocity if this is what your wanting to get.
I would develop a load, what I call a ladder load by increasing your compression from zero to what you feel comfortable with. Increase your powder load one grain using 3 to 5 rounds mark your cases for record. Like 3 68 gr next 3 69 gr to the point you don't want to compress the powder anymore. Usually 21 shots fired will give you an idea what can be used to improve the load. (wads, powder,primer or different grade of powder) This will also give you time to get comfortable behind the butplate and controlling the rifle. In other words "trigger time"