NEVER add percents to a 50 hit charge arms. I wouldn't add any attributes to arms unless they are like 75 hit, maybe a little less. This is because when you sphere something you will not get the value of the spheres back if you sell it. (Selling an item you have sphered will typically be sold at a loss.) So, if you sphere a 50 hit charge arms and then find a 60+ hit, you will be very disappointed by the waste of all the PDs.
Baranz is a much safer bet, because it can only have a max of 50 hit. That means when it hits 50 hit, you can not obtain a better one that will replace it. Thus, it is a save investment as far as spheres go.
And if your charge arms is Machine and hit, then you can only pick one other attribute to sphere it. You cannot add Native and Dark to it.
Lastly, you can't use photon crystals to add attributes to your charge arms. Attributes are added in Gallon's Shop using Photon Drops.
Edit: I'd also highly recommend not doing Native/Dark for your 1st Baranz. If you are only going to have one, you should do Machine/Dark, as the extra ATP will help much more there. If you plan to finish a set, N/M and A/D are much better for handling mixed spawns in Ep 2/4. Imo most of the mixed spawns that are N/D and A/M are not worth sphering for because a lot of those enemies can be killed by Hell easily. I really think that N/D and A/M is the worst composition for Baranz attributes.