Assuming they are intended for competition.

I would load all 300, and shoot them as a lot.  Next time around they will have more capacity and sight settings will be off slightly from the first loading. 

No matter every time you reload another lot of match cartridges sight settings have to be re confirmed anyway.

This for match shooting high round count, I run 500 38/55 cases this way. Half in Green MTM boxes other half in Red. Get to the end of a lot of 250, load up and test the other lot ready to go for the next round. Use up the remainder practice.  I never shoot matches with untested or mixed lots of ammo.

Boats

Instead of 'rattling 'em off' with smokeless, I would use them for some serious target work - after all, it is trigger time, and the stuff will most probably shoot pretty accurately.  Also, BP will better from the case - much better than an smokeless load.   
Any 300 rounds of practice must be good - so make sure you have some practice with those new cases.  A good way to learn about your rifle and your shooting.