In such a small case it will be very sensitive to bullet seat depth changes too. In just a tad more will boost pressures, so keep it consistent. Some other calibers use some 'tricks' to lower launch pressures, like the long throat in Weatherbys, so dont seat the bullet too long and jam it into the rifling, give it a little start room. With those considerations in mind go with the 75% of capacity idea for starters.
BTW, I subscribe to the 'overstabilization is a myth' school of thought. The faster twist is something the Hornet (and some other cal.s) have needed for years. The original 1:16 was on the edge with 45gr. bullets and they are like about a 115gr. in a 30 cal. Nobody shoots those in matches, do they?
Not only will the faster twist allow you to stabilize the longer, thus heavier, bullets with a much better ballistic cooefficient but it will allow you to shoot at subsonic velocities with much improved wind drift factors (but I suspect you already know this, that why you are building this rifle up).